Good day, could you clarify a couple of things? First, the Rega P7 is a discontinued model, are you considering buying second hand privately, or from a dealer? What cartridge would you use on the turntable?
The Parasound is a fantastic phono stage, no need to make excuses for the marque. I very much like your Rega/Parasound direction. Oh, and after a quarter century posting here, I am certainly no expert. Beware of experts!
There are some broad, general, challenges when using subs and phonographs. It is important to site the turntable so that acoustic feedback is not present, either structure borne, or air borne, and this can take a bit of experimentation.

