Subjectively OK or measured OK, as in flat response and good-excellent decay characteristics? Don, I also get good bass by simply using my F3@ 24Hz mains placed for best imaging (they do have some built-in filtering adjustments), but I can do much better by using an ideally placed, phase and FR-corrected sub.
Forgot to mention that the other benefit of crossing over the mains is that the bass drivers, freed from reproducing the lowest frequencies, suffer less distortion in the upper bass.
1. Both. I use RPLusD and a measurement mic (Earthworks M30) for my personal measurements; I grab equipment from work when I need to go further (rarely, now). My AVR and sub do have (independent) phase compensation, a topic I touched on elsewhere and agree is critical to matching sub and mains. When I have tried different sub positions, I have been able to get essentially the same FR after all the calibration and compensation is done no matter where I stuck the subs. I am less familiar with waterfall plots and have not yet used them extensively (yet -- stay tuned); my initial trials to date show similar results with waterfall plots as just basic FR plots, i.e. I can make them look decent no matter where I stick the subs. After some tweak time, mind you, with lots of measurements and listening.
2. My mains are Magnepan MG-IIIa's and, while old, are quite clean in the bass region, though certainly do not play as loudly as my subs. I am crossing over to the subs (a pair of Rythmik F12's) at around 40 Hz, just a little above the Maggies stated -3 dB point (but closer to where I measured the 3 dB in-room). The speakers I had before, a pair of Infinity Alphas, sounded better with the sub offloading more of the bass, and the system just before that was a sub/sat system (Mirage OMD5/Nanosat/10" Mirage sub, forgot the model). In general, I agree with your statement, just thinl 'most everything has exceptions...
In my case, remembering I am coming back into audio after a long layoff (and pretty new to HT, and least in a quality sort of way), my past life experiences led me to believe in having stereo subs in close proximity to the mains for the best image and cleanest overall sound. I feel the music is best served when everything is lined up at the source, the source being the speakers in this case. The times I have moved things around (i.e. tried the subs in different positions) I have not liked the overall sound as well even though frequency response was fine. Impulse response (measured) was somewhat trashed because it's hard to really line up the wavefronts with the sub and mains when they are physically separated, imo. So, even though I may or may not "feel" the waves in snch, I prefer to set up my system that way.
I participated in all sorts of tests (sighted, blind, double-blind, ABX) in my youth through a series of luck and being a pushy young kid. One the tests was an attempt to see where people localized subs. I don't recall everything, or even most, but what I do recall is that at 50 Hz virtually nobody could tell where it was; at 120 Hz everybody could; and, between 50 and 100 Hz or so was a continuum, with different people successfully (statistically) being able to localize the subs at different frequencies in that octave'ish. With THX movie the sub crossover to 80 Hz (about where I could localize it) and taking the range up to 160 Hz max, I felt it better to use a stereo full-range pair with the subs right there with the mains.
My final comment is one I should have placed earlier: I am not using the sub output of my AVR (Pioneer SC-27), but rather running the subs in parallel with the mains to create what I (and I think Steve and others) have called a "full-range-plus-sub" configuration. This is the way I last ran them, but then I biamped (active crossover) the Maggies plus used a pair of (custom, with a servo loop controller of my own creation) subs. I did not try the "normal" way and so cannot say if my way is better or not; it's the way I have done things for ages, and am too old/lazy/weak (take your pick) to keep moving things around and changing the hookups (HDMI has only slightly alleviated the rat's nest behind my console, and I get tired of having to undo and re-dress all those wires every time I try something new).
Sorry to be so long-winded, but now you know where I'm coming from, for better or worse. I certainly cannot claim my way is better or worse than anybody else's, but it works for me.
Or so I tell myself - Don