I'd be careful leaving the 950 at max volume, assuming you'll want to switch to another source in the home theater at some point. Could be a bit startling.

It's interesting that you are noticing such a difference with the signal still passing through the 950 - since the 950 is still in the signal path, I would think the difference in sound has more to do with settings (bass management, etc) than the 950's hardware. I'll assume you are using stereo bypass on the 950 for whatever input you have the Sansui's preamp outputs connected to. In that case, the DVD player's DAC's will be handling the digital-to-analog conversion, the Sansui passes that analog signal through it's signal path (adding any tone controls requested) to volume control and then on to preamp outputs, and the 950 takes that analog signal and passes it through a second volume control (this one leaving the signal largely untouched) and out to the amp. Two things occur to me at this point. One, you may be preferring the Pioneer's DAC to the 950's DAC for CD playback. This is not unheard of. With my Yamaha CD changer and Panasonic DVD-Audio player, I tended to prefer the 950's DAC, but with my Yamaha universal DVD player I prefer the player's DAC to the 950's and therefore use stereo bypass for CD listening. Two, a stronger bass signal may be noticed in this case if you have the mains set to small with a fairly typical crossover point (80Hz or so). This is because of the way the 950's stereo bypass behaves: the signal to the mains is passed unaltered (as expected with an analog bypass), but a copy of the signal is processed through the digital bass management and a signal produced for the subwoofer. The logic here is that if you have the mains set to small then the crossover is going to be set appropriately to cover the territory below the speaker's lower limit and allowing the sub to share that signal will help fill the void.

Here's an experiment to try when you remove the Sansui (or before, if you desire): connect the stereo analog output of the DV-563A to an input on the 950 and try listening to a CD in stereo bypass. You should hear much the same thing you are hearing now, with the exception being that any tone control introduced by the Sansui will not be present.