Subwoofers that do both L/R & LFE simultaneously?

I'm not quite as familiar with the RELs as you apparently are, though I'm not a fan of sending speaker level signals to subs.

I do know that both the Martin Logan Depth and Descent subs (I have a Depth) include line level (and speaker level) inputs for 2 channel; as well as a separate, single, line level LFE input; plus an LFE output for daisy-chaining to another sub if desired.

When driven by the LFE signal, the sub's contouring facilities are bypassed allowing crossover point, level, etc., to be determined by the HT processor settings.

I'm just not sure I understsnd what you mean when you say "the unit apparently senses the signal"? Do you mean that if you send the REL a line level LFE signal, it knows to stop reading the high level 2-channel signal? Cool, (I guess,) but you wouldnt need to worry about it being able to do that, if you just hooked the sub up with all line level feeds -- which as I mentioned earlier is best for sonics IMO (i.e. not passing the speaker signal for the main speakers through the sub.)

Just my opinion of course, but adding two more (mechanical) electrical connections and another pair of speaker cables between my amp and my main speakers just doesn't say "high end audio" to me. It's OK for a modest HT system, but at best a compromised result for 2-channel audio.

Nrenter, what you describe is technically the same circuit as I describe (except tied together at the amp instead of at the sub) And it's a little better, yes, for the same reason bi-wiring can be. But with the scheme you describe, you could really be buying lots of speaker cable!

Either way, the sub is sampling the amp output through a filter circuit of it's own. I guess that's OK, but being the stickler for cleanliness and simplicity that I am, I insist the sub have its own direct line-level feed out of the preamp.

Neutrik makes all kinds of connectors. XLR is only one of hundreds of types they manufacture.