Questions never stop ever, it seems.

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. 

@classdstreamer

I assumed that the line-level out from the sub was for daisy-chaining additional subs (for a DBA). Maybe this is sub-specific? 

This is correct. IME most powered subs do NOT have high pass capabilities. It is by far the exception than the norm.

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.

This is also correct unless the designer added a low frequency output. If you want to run a DBA it might be simpler and more cost effective to incorporate an external crossover and use subs that do not have a high pass filter out.

If I wanted to run a DBA, would I need an RCA splitter at the preamp?

This should work if you are using a sub that has a high pass filter