I agree with Geddes and this he is one of the few out there with the right idea.
ATC has always agreed with "don't insert another filter in a very audible place" idea when adding a sub. Our efforts on the pro side with multiple subs have worked every time in solving room bass issues, especially avoiding bass build up on the back wall (behind your listening position) and nulls in the space that you cannot resolve due to console placement (which would parallel a sofa position in a home listening room). So we follow the plan that multiple, smaller, lower SPL level subs located on different walls (if at all possible) seems to give us the most even bass in the listening space.
What appears to be more important than high pass filtering the mains (which inserts a very audible dip in response at 80-125 and gets worse with distance of mains to sub) is adding delay to the mains. This part ATC does not agree on inserting a DSP across the entire audio band to accomplish this mains delay, as you can "hear" many of these DSP platforms. This is why they stick with an analog input to not force you to listen to a permanent unchangeable DAC. The DSP effect is certainly as audible as cable differences, or differences between DACS, CD players or other digital sources.
The reality is that in many cases the negative sonic effect of the DSP platform may be less than the negatives of not using delay at all or filtering the mains and creating phase issue.
Brad