I just did this with my center channel, so I thought I'd add my experience to the mix (...as it were...
The way I have it set up is as follows:
--- Receiver ---Onkyo TX-NR807
CC speaker-out (xover at 40hz) -->
--- Sub ---
(right) speaker-in -->
Mirage LF-100 (right) speaker-out -->
--- Center Speaker ---
I'm still tweaking the sub's level and crossover settings, but right now it's at about 60% level, and I have it sending frequencies over 90hz to the OMD-C1.
I really like the result. I'm running the system without a sub
right now, but telling the AVR it has one, so that it will allow me to set the fronts as not
"full range" -- which they aren't, and so would possibly damage them to send that signal.
It lacks the gut-rumble that a sub would obviously provide, but it definitely
has that nice "mid-bass" impact that the Center/Fronts were lacking, being bookshelves. I recommend it if you have highs-leaning speakers, and an extra sub laying around.
One question: Should I keep the sub paired to the center channel, or should I pair it to the L&R speakers (it has left and right I/Os). Related, should I "bi-amp" the center, and have the sub's L&R outs both
go to it; or should I leave it with only the right channel in and out?