This answer may seem a bit confusing, but I've tried to cover all the bases...

With the setup you describe, the 3910 is redirecting .1 to the mains for 5.1 material, and with the 5.1 switch off the full range speakers undergo no change (although the low-pass filter to the subwoofer on the 950's analog bass management is always active, so a copy of sub-80Hz material is being sent to the sub). In the case of a two-channel source being played through the six-channel analog input, the mains would still have no bass management applied, but the low-pass would once again send a duplicate ot the sub-80Hz material to the sub.

What about bypass on a stereo analog input? If the mains are set to "large" in the 950's digital bass management, then the stereo analog bypass applies no bass management - full-range signals go to the mains, and the sub gets nothing. If the mains are set to "small" in the 950's digital bass management, then stereo bypass still sends an unaltered full-range signal to the mains but takes a copy of the signal and applies digital bass management to generate a signal for the sub.

Bottom line: stereo bypass and the six-channel analog input will behave the same for a stereo input if the analog bass management switch is off, the mains are set to "small," and the crossover point for the mains is 80Hz. If the mains are set to "large" or have a lower crossover point, the sub will get less data (possibly none) with the stereo bypass.