As long as the bass management is done in the digital domain there will be no phase errors. That's one of the beauties of digital filters. Check out this technical article for a good explanation. The discussion is centered around CD player "brickwall" filters but the concepts are the same when applied to digital filters implemented as a crossover. The most commonly used type of digital filter for audio use is the FIR (finite impulse response) which is further discussed here . This not to say that digital filters are perfect, they have their own set of problems but the introduction of phase errors is not one of them.
Bass management done in the analog domain can introduce phase errors and having staggered cutoffs for the different channels could have some interesting results. However, in the case of the 950 analog bass management is only done on the 6ch input AND the crossover point is fixed at 80Hz for all channels. Therefore, any phase error introduced by the analog circuitry will be uniform across all channels. I also believe that these phase errors would be largely swamped by the effects of the room and speaker placement when considering the long wavelenths at 80Hz and the distances between the speakers and the sub(s).