Channel trim is most important for surround material. Think about a two-channel receiver - do you have left/right trim control? Granted, balance control could be considered to do something similar, but I suspect what the 950 is doing is bypassing the trim control along with a lot of other processing when it is in stereo bypass mode. The six-channel input would need to still strongly benefit from having the delay and trim adjustments applied, which is why you are still seeing trim adjustments affect output on that input.