Can i plug an amp line out into the preamp line in, then run the preamp out back into the poweramp in? Or will that create a blackhole?You certainly should not run an amplified out from your speaker taps on the amp back into a line level input on a pre. I used to do that and called it "wiring amps in series" but it's a seriously dumb thing to do and it doesn't increase power or do anything else other than impress your college roomates that you can splice a speaker wire back into an RCA jack.
If you are talking about a line out - say a tape loop or preamp out, you can do it. Depending on the topography of the gear, daisy chaining all this together could dilute the signal and degrade quality. And bluetooth is generally not considered quality.
One thing I like about Roon for digital is that you can use their zone functions to duplicate a signal through multiple DACs and still have them synched, which allows more tinkering.
Have fun! I second the idea of not drinking heavily while soldering, but there are of course other intoxicants that might enhance your experience.