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.)

The Wilson Watch Dog has a processor LFE input as well as a stereo line input,....but you have to switch between them as you change from HT to two channel....not ideal, but it does permit two setups. By the way, I find the Watch Dog to be a terrific supplement to Watt/Puppies...similarly at 30.

Ruark sub allows you to do 2 channel line level and LFE at the same time and has two gain controls, one for each input.

I'm not really sure about your question, but in subwooferdom two is better than one. Never heard negative experiences with two rather than one sub. This guarantees you a balanced stereo bass. I used one sub for years 'til I switched to two, from Audiogon suggestions. I was released. As was my system. peace, warren