Questions never stop ever, it seems.

 If it's crossed at 80 and a 6 or 12 db crossover is used, it is still going down to 30 hz.  The taper on the roll ON (not off) is still from 20 or so all the way to 80 and 80 begins to roll OFF from there @ either 6 or 12 db per octave. So running full range can work but most folks don't like the sound SIMPLY because the timing issue from the mains can't be delayed as easy as the DBA or Swarm system..

I've tried it with every system I've ever used or owned.. I gave up all the way 15 years ago. NO BASS in the monitors. It may be there but I don't use it OR I swap it for MB couplers..

Still you got BASS in the monitor cabinet.. It cannot be timed correctly..
The best you can do is DELAY the signal from the mids and highs...

I also understand the way people blend  their DBA is all important. The SQ is usually a lot better. USUALLY folks STOP there and call it good, because it is good. BUT for me I wanted BETTER.

No BASS in the monitors.. ZERO... NOW the monitors are 2/3 narrower because of it... AND they do actually disappear in a room a whole lot easier..

It is like being in the maestro position all the time not some of the time..

Regards

That is exactly how I have my system currently configured. Preamp Outs to Subwoofer In and Subwoofer Outs to the amplifier itself. That engages the hi pass filter and gives me some control options on the subwoofer but are you also saying the system is now bi-AMPED not just bi-wired? I'm just trying to make sure I understand here.

Yes.

You are using one amp, built into the sub, to drive frequencies from 20 Hz to 85 Hz and a different amp to drive frequencies from 85 Hz to 20k Hz. IOW you have broken the frequency spectrum (20 Hz - 20k Hz) into two segments ( Bi ) with each segment being driven by dedicated amplification (amped)

Hope this is helpful.

I have never considered running preamp out to the sub, then RCA from the sub to the amp that powers the mains. The lag time from the mains and bass does affect me. Something new to look into.

I assumed that the line-level out from the sub was for daisy-chaining additional subs (for a DBA). Maybe this is sub-specific? So if the line-level out from the sub engages a high pass filter, it seems you can't daisy-chain subs for a DBA. If I wanted to run a DBA, would I need an RCA splitter at the preamp?

On the topic of connecting the sub to both the high and low level inputs (amp and preamp), I believe I read that Rel could do that in a home theater application. That is, the Rel could both extend a center channel, or R/L channels, to full range while also being connected to the LFE. 

OP, I may have misunderstood, but I don't know how/why you would connect a sub via speaker wire (high level) from the mains power amp that you are simultaneously high pass filtering with the sub. 

On bi-wiring the mains. It seems to me that you could still bi-wire the mains from the power amp, which is receiving its signal through the sub.