... I dont listen to 7 channel stereo normally, however it is GREAT for parties. The bass level does turn louder in order to compensate for the additional speakers in the room playing, which is good. My previous Denon merely left the sub level the same, so in 7 channel mode the 7 channels will overwhelm the sub unless you manually turned it up to compensate.

I used the config mic to set the relative levels. Each channel except the fronts are set to small. The front speakers are being routed through an AudioControl Richter Scale III crossover. They are auto set to 0 dB (so they are not pegged to +15 db) while the rear channels needed some boosting because the amplifier isnt as sensitive.

I was playing a dance club mix CD (mainstream dance, not hip hop) in 7 channel stereo mode and when I got to -15, (yes room filling but anything but "loud") I heard this popping in my front channels to the beat of the music! If I turned it down, this went away. If I turned it louder, it sounded even worse. I looked to see if the amplifier was clipping but it was far from it. I really don't think its the AudioControl as I've pushed that thing way further before. I then removed the speaker grilles to see if the woofers were being bottomed out, but they only moved just a bit. I changed the mode to "stereo" "bypass" and "upsample" so only the front channels played and was able to turn the 990's volume to +5 before any distortion. Overall the sound was MUCH louder even though only 2 channels were playing. I think something inside the 990 is being overloaded.

Does anybody else have this problem?

Do you have a sub? I'm making a guess here (since I haven't run into it myself), but it sounds as if all of the summed bass from other channels is being steered into the mains (where it joins the low frequency material already there for the mains) and begins to overload the capabilities of your mains.

I did some more playing around I think you're right... the difference between 7 ch and 2 channel for the bass is ENORMOUS. I watched and my amp that drives the fronts clip lights were flickering.

Here's the thing about 7 Stereo (at least from what I know of it): you're taking two signals and copying them - the surrounds see direct copies of the mains rather than the typical "background" data they get, and you are getting the equivalent of setting up three identical left speakers and three identical right speakers. The additional sources are going to yield a higher volume level (I'd expect somewhere around 6dB, since each channel ought to pick up about 3dB for each additional source). When you then toss in bass management of all of those channels, a similar increase in signal magnitude gets sent to the sub (or to the large mains, in the absence of a sub). The result is a serious jump in SPL's at the same volume setting.