@erik_squires, I personally appreciate the benefit of integrating a sub into a restricted low-freq. system, IF the sub can be seamlessly integrated. As most of you knew, the sub is not only filling the low end but could also augmenting soundstange, dynamic range, etc. When listening at low volume, the sub also fill in the low end atmosphere just like the "loudness" button.
Nevertheless, integration of sub is not an easy task. I completely agree with you that high-pass filter (HPF) is almost a must after experimenting one of the few subs that includes such a function, i.e., SVS SB 1000/2000. The HPF in SVS SB is fixed at 80Hz just as mentioned in your article. I am not sure why SVS abandons that in the pro model and higher grade sub. With HPF, the headroom in both main speakers and subs becomes bigger and enables them to handle the notes with more authority. The integration is much better than other methods of connection, such as high level, line level, LFE, or from speaker level to line level through a line-out converter/attenuator. I just wish the amp in SB 1000/2000 could allow users to adjust the Q setting or the HPF frequencies at least has more preset frequencies bet. 60 Hz to 100 Hz like in Emotiva.
Like you have mentioned, the DSP method is another way to integrate a sub. However, the more affordable digital signal processor like MiniDSP does not have digitial OUT that allows users to connect it with the DAC. I am not confident the quality internal DAC in MiniDSP. If placing the DAC before MiniDSP, the MiniDSP will need to perform A-D-A conversion internally and potentially mess up the quality of digitial/analogue signals. To be completely digital, one need to upgrade the DSP to SHD Studio which is sort of cost prohibitive around $1k. I am not sure if you have the exposure to DSP and could share your experience/advise.