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.