Well after finally getting it to work, I can report some very positive results, the most noticable being a taming of my sub and some boominess which made high volume listening uncomfortable. I can now listen at higher volumes. There is plenty of scope for tuning, so I suspect that with some effort I will be able to limit its impact to something like what MRC does, though I haven't heard any pre-echo effects of the sort described by M for this kind of filter.

Meanwhile I would recommend this to anyone who does not have MRC (and a PC source)

HH1 Photo 34. 598DP, 504, 518, 561(M), DSP5000, DSP33C, DSP33
2nd Room: 541, 555