So-called 7-channel or 5-channel "stereo" is probably the hardest mode on a receiver, since you are sending identical signals to multiple speakers. The left stereo channel goes to the front left, the left surround, and (if equipped) the left back surround, the right stereo channel goes to its 2-3 speakers, and then a mono-mix goes to the center channel. This is harder on the speakers than most discrete 5.1 source material, where the surround channels are more or less uncorrelated to the front and center channels, and also tend to be at lower power levels. Since the transient peaks (in 7-ch stereo mode) occur in multiple channels simultaneously, you are also more likely to overtax the receiver's power-supply, which may not be capable of delivering full rated power to all channels simultaneously, when they are highly correlated.
You might want to try sticking to straight 2-channel stereo, or else using something like PLII (or PLIIx) for derived 5.1 or 7.1 playback from a 2-channel source. Although 7-channel stereo fills the room with sound, it also is a pretty artificial effect, at least in my opinion.
