I'm using a Denon 4800, and it clearly sounds better in stereo mode than in surround. The amplifier section is rated at 125W/channel with all 5 channels driven, and 165W/channel with only two. So my first instinct is to say that the extra power is what's helping. But, the THD also drops when placed in stereo, from .7% to something like .1% (can't remember the exact number). My speakers are 91db efficient, and I don't listen at ear-splitting levels, usually around the -20db level according to the Denon. So maybe it's the lower distortion that sounds better? I'm planning on buying another amp, but I don't want to pay for 200W/channel if 100W will give me the sound I'm looking for. Do I need the extra power, or just an amp with less distortion?