A little late for the original poster, but nonetheless...

I was having the same problem with my new 990, configured for 4.0, generating way too much bass when listening to stereo sources and after more research I think I understand what's going on now. The short answer, perhaps counterintuitive, is that you can fix your problem if you go into the Speaker Size configuration and set your subwoofer size to "LFE only".

Here's what I think's happening. If you think about what PL II or DTS does when it's creating surround from stereo, it's generating the center, surround and subwoofer tracks from a two channel source, essentially out of thin air. After doing this, the 990 routes the (newly created) .1 LFE track to the subwoofer, but if you've set your subwoofer size to "NONE" then that LFE track gets mixed back into your "large" speakers, effectively adding in extra very low bass.

But if you instead set your subwoofer size to "LFE only", the 990 sends the .1 LFE track to the subwoofer jack instead of mixing it into your "large" speakers. This track is then lost (since you don't actually have a subwoofer attached), but that's what you want. CDs sound fantastic in surround here with this setting.

Remember to set the size back to "NONE" whenever you're listening to true 5.1 (or better) source material (e.g. SACD, DVD-A, or 5.1 movies) or else you'll be missing bass in those cases.

It's a pity the 990 can't do this automagically, but perhaps the real fault lies in the definition of the music surround modes for PL II and DTS because that's just way too much bass for most stereo sources.

I am also having this problem (see my post regarding 990 recording). It seems any analog signal I send has greatly increased bass. I called Outlaw and they stated that in the stereo mode, using an analog signal, the subwoofers are fed a full range signal. I guess bass mangement is different for the analog signal. One solution is to activate the crossovers in your subs, if you have them. However the problem with that is that you now have two sets of crossovers (990 and sub) interacting. One change begets another. However, I was able to get a "useful" compromise doing this. However I am going to try something different. I have speaker level inputs on the subs. I can run speaker wire from the 990 to the sub and use the subs crossovers to pick off the bass and then send the rest of the signal via speaker wire to the mains (I have def tech bp 20's and two def tech subs). On the 990 I am going to set the fromt speakers to large. In effect the subs/main are running as one. Hopefully this way I can obtain proper equalization from both analog and digital signals. I asked outlaw if this issue was something that might be fixed in a firmware update and the response was no. It was hardware related. This is a complicated issue and hopefully I did not mistate anything regarding my conversation with Outlaw. If anyone has a different understanding of how the system works please post.

What appears to be happening is when you have processing on (Stereo and Upsampling etc) a sub signal is being generated for the nonexistent sub and then since you have sub off it is going to your mains. Derrell's workaround above may do the trick. Outlaw has said they are working on a fix for some bass management issues.

My experience with the bass issue is very different from what you guys have posted. I only play stereo music CDs, and never use sound processing like DTS or prologic II, etc. I found that the bass in Stereo, Bypass, and Upsample modes are basically the same. This is probably because the DSP in my CD player (Rotel RCD 1720) is as good as the one in Outlaw 990. However, the bass in the subwoofer becomes much louder and sounds better (deeper) when I switched to the 5.1 channel stereo. Why is there such a big difference in the bass from subwoofer between Stereo/Bypass/Upsample and the 5.1-channel Stereo modes?

In the set up menu, I have my front L/R speakers (Dynaudio Contour S1.4) set to “Small”, crossover to 80 Hz, and Subwoofer to “L/R+sub” . A center speaker is also enabled and connected. But I do not have any surround speakers connected. The front and center speakers are connected to Outlaw 7500 power amp. Analog signals from Rotel RCD-1072 CD player are feed to 990 preamp. The LFM-1 subwoofer is connected the subwoofer out of 990. All cables were from BlueJeans.com. Balanced cables are used to connect Outlaw 7500 and 990.

If you don’t have a subwoofer why not just use Bypass mode for 2-channel listening? The only reason I can think of for using the Stereo or Upsample mode is if you want to engage the bass management feature of the 990. Since you have no sub there would be no reason to use bass management in 2-channel mode. The original question in this topic was about the different 2-channel modes. What about Dolby D and DTS? Do they work as expected when you do not have a subwoofer in your system?

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Your stereo sub setup is not extremely common, but it's also not unheard of. I believe that your issue is one of the bass management problems that Outlaw is planning to resolve with the firmware update. We don't have any information on when the update will arrive, but it is in the works.

TommyBoy

You bring to light something I thought in my mind but didn’t give much thought. My main speakers are full range but I also have a sub so I set the fronts to large and the sub to L/R+Sub. I thought it strange that you could still set the Xover for the mains when it was set to large. The Xover setting should not even be available when set to large. I would say this is a design problem. If I had known about all these flaws in the bass management of the 990 I probably would not have purchased it. I am still in the 30 day trial period and am considering sending it back because of this problem and some others that bug me about how the 990 works. I wish I knew what fixes they intend to include in the software upgrade they are working on.

First let me say I am competely satisfied with My 990,in my configuration. I use stereo subs paired with L and R speakers, a third sub driven by the 990 sub Out, and all speakers set to large and sub set to LFE only. However, with that said I think it is time for the Outlaws to publish a Matrix of all possible speaker setting with the crossover effects for all inputs. I was truly amazed to find that crossover setting worked on speakers set to large, when sub is set to LR+sub. I agree with the above post, crossover should not affect speakers set to Large,EVER. Setting speakers to large is the only way to bypass the digital processor in the 7.1 direct mode, but it is not in the manual, it just got passed along by word of mouth.
I am ready to see a matrix of how the 990 operates now , vs how It will operate with the new software. There has been to much speculation and work-arounds put forth on this forum with no real input from the Outlaws. IMHO the lab and beta test plans were poorly written or not followed. Or maybe there is some sane reason for the way it seems to operate. I am not asking for software tomorrow, just something that will clear the air on how this highly complex pre/pro really works. The datas for such a matrix has to be allready available and should be easy to publish, while not delaying the proposed software fix.