Two basement concrete walls, and two drywalled 2x4, one open stairway. Open wood floor joists 7.5 ft tall. Room dimensions are 21x27 but I'm only using about 14 foot main speaker spacing. Danley SH-50 mains. One in corner one not. Both on top of TH-50 (like Tom Danley's home setup, I asked him). SPL TD-1 (Danley's orig. patents) surrounds. One TH-50 in rear to balance. Audyssey XT/32 room correction. I measure 3W peaks on mains at 85-90 db listening. The only time I come near clipping on the K2 is when I put on Edge of Tomorrow as my "torture test." I don't use any Highpass filtering other than the 8 Hz. in the Crown K2. Very simple...one receiver, one sub amp. for 6.1 and 2.1. Thanks for the checklist of factors, Mr. Ricci.
My own conclusion is that that even though response below 18 hz. falls off more quickly the EMOTIONAL and gut shaking impact is still there on the best Blue Rays while retaining musicality to 2.1 since I use the same setup for 2 ch listening also. So I do consider bass below 20 hz. to have diminishing returns. I was seeking other comparative opinions.