Does the 990 used in a 5.1 channel config allow the two unused channels to be used by the mains for bi-amplification.
I am using Axiom m80's
Thanks in advance for any info.

Hi mcleod, I'm not a 990 owner but I think I can answer. I'm assuming you have enough channels in your power amp to do so. You can split the output of the 990 for each channel you want to biamp by using an RCA Y-cable.

The 990 doesn't have internal amps - if you want to bi-amp, you'll need a seven-channel amp and a Y-cable to split the left and right pre-amp outputs.

If you mean active biamping I think you'd feed the two front signals to a separate line-level crossover network and feed the four outputs from that to four amplifier channels, on a seven-channel amplifier, two (preferably identical) stereo amplifiers, four monoblocks, or whatever, each of which would connect to one of the tweeters and woofers of your main speakers, bypassing the speakers' internal crossovers. The center and left and right rear outputs of the 990 would of course each feed another amp channel which would drive the corresponding speaker. The 990's left and right surround channels wouldn't be used. This is more straightforward than it sounds. I'd be interested in the aural benefits.

That's a good overview of bi-amping, psyprof1. The active bi-amping approach (using a crossover network between processor and amps, with the crossovers removed from the speakers) can yield some real sonic benefits, but it is dependent on getting the design of that crossover network just right. Very few of us can get anywhere close to taht. The passive bi-amping approach (what Hoosier Benzo and I were both thinking of) is much easier to achieve, but the benefits are less significant - I've heard it compared to using a single amplifier with a total output equal to the output of the two passive bi-amping amps. In the case of mcleod's M80's, we need to determine what amp is being used - specifically, if it has two extra channels and if the extra power would be beneficial. I doubt that bi-amping using two channels of a 7700 per speaker would be much different than using a single channel of a 7700 or 7500 for each speaker, but a 7075 would be a quite different story.

Good dialogue. I agree with gonk that biamping using 2 high power amp channels (200WPC or more) would probably not yield much benefit if any over just using 1 channel of same type.

I'm considering the 7125 and do not need the extra 2 channels since I'm running 5.1 only. I'd probably biamp simply 'cuz I can'!

I run fairly power hungry PSB Stratus Silver speakers (4 Ohm-ers), so there might be some actual benefit there too.

There may very well be some benefit to the extra power for low-efficiency speakers, but there's also the fact that the 7125 can provide 190 watts to 4 ohm loads.

Haha point taken!

But c'mon I'm CONSTANTLY exceeding 190W continuous output on program material.

Did I mention my listening room is a football stadium?

I'm passivly biamping now because I can!!!