Huge topic:)
I would think it through this way ( assuming P6 is staying )
Cart that is a good match to Rega arm > phono that is good match to cart / system gain.
This means ( to me ) a relatively light weight MC, one that can be set to a decent SRA/VTA with spacers ( assuming you didn't mod the arm with a vta adjuster ) and then ( since you like tubes ) a synergistic SUT + Tube phono.
Of course budget range needs to be at least loosely defined to get more specific.