Regarding the gap between the low range of the speakers (on paper, 100hz) and the high point of a sub (no higher than 80, or maybe 70) - is there an easy way to measure this? I could use the REW program, or Audacity, I think, but I am looking for some easy-to-use phone app that can tell me about the output in a specific range (like 80-90-100 hz). Ideas welcome.
Another way would be to use test tones, and our trusty old friends, the ears. I found some sine wave tones on my pc. The FR2 plays tones down to 125hz very loud and clear, no problem. They play a 100hz tone clearly also, providing I turn up the volume (confirming what reviewers have said). Surprisingly, they play 80hz and even 63hz tones also, still clear, at the same louder level. It is only when I play 50hz that they give up, I hear ca nothing. So this would suggest, I think, that the "gap", on paper, between speaker and sub limits may not actually be a problem. My general impression, using subs and main speakers, is that the main speakers should not be cut off at the bottom end, unless there is dire need for it. I think this goes for the FR2 also. Maybe I am too tolerant of "colorations", "doppler effects" and other suspects, but I find that full range speakers should be given a full range signal, to sound best.
Perhaps the ideal, in my context, would be a small sub that has speaker level connections - avoiding crossover/equalizing - that helps the speakers along in the 100 or so hz to ca 60 hz range (or however low it goes). I have some flexibility and can position the sub so it just helps the speakers along, adding some more deep mid and upper bass tone, without disturbing the clarity of the speakers. Positioning + subtle use of sub volume (so it stays "invisible") may do the trick.