I've got a couple of monoblock amps/xlr hooked to my 990 & I'm thinking of buying a couple more to bi-amp my front L/R speakers.

I searched the forum & it looks like the suggested method is to use XLR splitters. Is this the case?

Is there not an option to utilize the surround back left/rights as part of the bi-amp system?

Lastly, if the only option is to split the XLR from the left/right to 4 monoblock amps, will this improve the sound quality? Or will it not give me anything in return? Or is this subject to interpretation of the listener?

Currently, I use 2 tower fronts (Polk Monitors) & a Polk powered subwoofer. I've been pretty happy with the 3.1 surround so far. :-) The main thing is that I don't think I have the space in my living room for a full on 5.1 or 7.1 speaker system, but that may change. I already have all the speakers for 5.1, the surround backs & center are being used in other stereo systems, just didn't get all the monoblock amps to use on the Outlaw...monoblocks were getting expensive, but I'm finding some good deals now).

No. The Model 990 does not allow re-assigning surround back outputs. You find this feature on receivers because of the power limitations of built-in power amps. With a surround processor, the amps are whatever you bring to the table - rather than steal two pre-amp outputs that you may want to use, the simpler solution if you do decide to bi-amp for some reason is to employ a splitter.
There are a lot of variables involved. We are talking here about passive biamping. The only real benefit here is more power, something that could also be achieved by employing a single larger power amp. And if the existing power amp is large enough that you are not pushing it close to its limits by itself, having more amps may not achieve anything sonically.

Thanks. That pretty much answers that question.

There have been many debates on the subject of passive vs. active bi-amping (or tri-amping, or ... ) in the Saloon over the years, and you can search to find some good, bad, and ugly threads at your convenience.

I have mostly respected those people who believe that only employing the crossover network before the individual amplifier stages (active bi-amping), and bypassing the internal crossover network in the speaker (if one exists) will have positive results. Also, I have found that many of the arguments in favor of passive bi-amping are based on bad science or just unsupported opinion.

I do, however, bi-amp my B&W N803 mains (with the internal crossovers intact) and have been very satisfied with the improvement.

The main benefit for bi-wiring, as I understand it, is to isolate the interaction of the Low Frequency drivers and the Midrange/Tweeter. The movement of a traditional coil/cone driver causes variations in the impedance of the circuit at the speaker end of the cable, which in turn can create harmonics that are reproduced by the other drivers. Bi-wiring eliminates this interaction.

Also important to consider is the power demanded by the Low Frequency drivers is many times the power needed to push Midrange/Tweeter drivers to the same Sound Pressure Level (SPL). Any interaction between these two circuits will cause distortion of the amplified power applied to the HF (midrange/tweeter) circuit and will have a impact on the sound quality. The HF drivers provide the voicing (tonal accuracy) and the spatial information of the speaker. As amplifier induced distortion tends to rise at higher power levels, a dedicated amplifier for the HF circuit can operate at lower power and at it’s lowest rated distortion. The amplifier pushing the LF (low frequency) circuit is free to push it’s limits, while providing a better damping factor and therefore providing improved control of the bass drivers.

For those, like myself, who use EQ (equalizers), there are other benefits. In my setup, while the HF (midrange/tweeter) circuit is essentially run in bypass mode, the LF (base driver) circuit is run through a parametric EQ between the pre-amp and the amp. This allows for the correction of room/speaker characteristics, without getting the HF signal involved, where EQ distortion would be more evident.

Considerations:
Some important questions to consider when approaching this fork in the audio road are:

1) What are the specific characteristics of the speakers, and do they easily support bi-wiring or bi-amping with the existing internal crossover?
2) What are the sonic characteristics of the speaker’s drivers, and will they benefit from the separation of amplification?
3) What are the sonic characteristics of the drivers and room interaction at specific frequencies, and will this benefit from separate signal equalization prior to amplification?
4) If considering active bi-amping, do you have the necessary expertise and equipment to analyze and configure a successful active crossover network? Do you reasonably expect to map the drivers performance and develop a crossover superior to that designed by a high end manufacturer? And do you have the money and time?

Bi-Wiring and Bi-Amping:
(From the FAQ section of the B&W website)

Most B&W speakers are provided with two pairs of speaker terminals; this allows you to either bi-wire or bi-amplify them. The aim of both these techniques is not to simply get the customer to spend more on cables and electronics (although no manufacturer objects to this spin off) but to improve the resolution of the speakers.

A multi-way speaker contains a crossover network that not only divides the incoming signal into different frequency ranges, appropriate to the working range of each drive unit, but also equalizes each driver’s response to be flat (raw driver responses are usually anything but flat).

There are two different basic types of crossover – series and parallel. Series crossovers have each filter section wired in series between the positive and negative input terminals. It is impossible to treat each filter section individually – each interacts with the others – and such crossovers are not suitable for bi-wiring or bi-amping. But by far the most common type is parallel. Here, each driver has its own filter wired between it and the input terminals. If there is only one pair of input terminals, the inputs to all the filters are connected in parallel to that one pair of terminals. If, however, you have more than one pair of terminals, you can completely separate the inputs to each filter. Why on earth would you want to do such a thing?

In the case of bi-wiring, the answer lies in the cable connecting the speaker to the amplifier and the fact that the amplifier is a voltage source but the speaker is a current driven device (force on voice coil = magnet flux density x length of conductor in the magnet gap x current).

Firstly, all cables are a compromise. Some types of construction work better at low frequencies and others at higher frequencies. Providing separate inputs to the speaker allows you to use different cable types, each optimised for the frequency range of use.

Secondly, consider that the cable has an impedance that causes a voltage drop along its length. Now consider the current flowing along the cable. Assume for the argument that the amplifier delivers a perfect voltage waveform to the cable and the cable itself adds no distortion. However, each driver has a non-linear impedance (for example, the inductance of the voice coil alters depending on its position in the magnet gap) that causes the current to be non-linear. This non-linear current through the impedance of the cable causes the voltage drop along the cable to be non-linear and thus the voltage across the speaker terminals is also non-linear, even though it is linear at the amplifier end.

If we were just concerned with one driver, things would not be too bad. But that non-linear voltage at the speaker terminals may contain harmonics within the frequency range of one of the other drivers and that driver will reproduce them, albeit at low level. If, however, you separate the inputs to each driver filter, each driver’s distortion is kept to itself and the total system distortion goes down. We are talking small changes here, but the resolution of some modern drivers is now so good that small improvements like this are readily detectable by keen listeners.

Many people ask us whether the load on the amplifier is different if you bi-wire. It is not. As far as the amplifier is concerned, it matters not one jot whether you parallel the inputs to the filter sections at the speaker end or the amplifier end of the speaker cable.

Bi-amping takes advantage of all this and adds some benefits of its own. Like with cable, you can choose different amplifiers that excel in different frequency bands. You may, for example, be keen on valve (tube) amplifiers. But even the most die-hard of aficionados would be hard pushed to claim that they are any good at keeping good control of the bass. Bi-amping enables you to combine a valve amplifier for mid and high frequencies with the control of a solid-state device at low frequencies.

Unlike bi-wiring, the load to each amplifier is different from that using a single amplifier full range. The voltage demands on each amplifier remain the same (each is still fed a full-range input and gives a full-range output), but the current demands are reduced. This of itself can improve the amplifier’s ability to deliver the signal to the speaker.

Be careful when bi-amping that the gain and polarity of each amplifier are the same, otherwise you will compromise the frequency response of the system.

Copyright Disclaimer
Bowers & Wilkins home audio and home cinema speakers 2003

When bi-amping, be aware that audio power distribution versus frequency is not linear. Lower frequencies have a much higher share of the power distribution than higher ones. For a typical 2-way speaker with crossover around 2 KHz, the woofer section will require approximately 6 times more power than the tweeter section for balanced sound output. The 50-50 point is around 350 Hz. What I've seen done in many bi-amping arrangements is equal amps for high and low frequencies. This is a waste of amp resources for the high frequency section, and only makes a small improvement to the low frequency section. One should also be aware that only a few less common tweeters are capable of handling more than a few watts of power. Those most of us will typically encounter in a home hi-fi speaker are maybe capable of a steady-state 15 to 25 watts. Connecting a 100 or 200 watt amp and expecting to drive the tweeter to the power capabilities of the amp will likely blow it, unless a passive filter section (crossover) ahead of it dissipates most of that energy (not likely).

While there's nothing technically bad about passive bi-amping, it's one of those commonly marketed techniques that doesn't really add much bang for the buck. The unfortunate part is it distracts from areas where there can be big bang for the buck, although not always for the audio component retailer. I always find it interesting when I visit a forum thread debating some piece of gear, particularly a technically insignificant peripheral item, and then discover a room photo that shows an acoustical arrangement that will surely swamp any gain the item being discussed could possibly provide.

Bill