Would "rel style" be akin to an active Xover's functionality whereas one preamp out goes to the miniDSP and that to the coffin subamp and the other preout to my main amp out to my main speakers?
Yes, then the woofer is blended in with the lows the mains oututs already.
….and i do believe i have 1 or 2 subamps still sitting in a box somewhere
Dig them out, you have a cheap experiment coming. Ideally 2 the same, but just to find out one with the woofers paralleled to get you going. miniDSP and a dedicated amp just gives more flexibility.
Option 2 would be possible since i have an extra 400w amp in storage. Its been a while since i read up on bi-amping with active xovers. the signal from the active Xover is sent to both amps which deliver a specific frequency bandpass to each pair? this one specific bandpass, will be taken in by my main speakers internal Xover and processed accordingly by its own design with no sonic degradation?
How many of us don’t have an extra amp in the closet ;^)
Yes, that is the functionality (a miniDSP can accomplish this but i would not use the cheapest ones).
The active XO should hav elittle affect on the mains, it is after all goinf thru more electronics, but by relieving the mains from heavy lifting in the bass will improve the midrange (and the top of the mains is a single driver fullrange (what i mostly use),
Limited, but ast close to transparent as possible, would be a first order
PLLXO, essentially an extra cap in fron tof the HF amplifier.
dave