I'm pretty sure the double bass of the 990's bypass mode is a result of the L and R's bass being summed and sent to the subwoofer. I have "small" speakers that don't go much below 80Hz and the bass is significantly louder in bypass mode. I imagine if someone was using full range speakers up front the bass would be even more pronounced as they would produce bass as well. In that case it might make sense to turn off the sub when in bypass mode.

If the 950's DSP is active for the sub I imagine it avoided doubling the bass. If that scheme was used in the 978 it would be an improvement over the 990's bypass mode. Maybe a 950 owner that can confirm its bypass operation.