It's worth noting that once you change the OT, PT, the phase inverter to a long tail and maybe the speaker to a 12" you really no longer have a PR. Especially the OT. I tried one of those BillM TO20 PR replacement OTs that he developed with David Allen in my PRRI. It did help the low end fartiness like they said it would but it also changed the character of the tone in a manner I did not like. I put the original OT back in. I bought a second PRRI. One of those "Navy Blues" FSR runs with blue tolex and a Celestion Alnico Blue 12" speaker. It's a fine speaker and I love the amp (I run them in stereo - the other one has a Celestion Alnico Gold 10") but side by side it's not quite as "Princton-ey" as my amp with the 10". All of the suggestions above with lead your PR in the direction of the DR which is no longer a PR. I think the concertina PI gives the PRRI much of its charm.
I should also mention that my original PRRI is also an FSR run. One of the ones with the tangerine and white tolex and a Jensen P12R Alnico 10" 25W speaker. Once I bought the blue PRRI with Alnico Blue I pulled the trigger on the Alnico Gold thinking it would be a substantial improvement over the P12R. It wasn't. It's a great speaker and I do like it but it was not a massive improvement over the IMO very nice P12R. It would probably make for a nice upgrade to the stock Jensen ceramic 10" speaker. I guess what I am trying to say is that you have to weigh all of these changes individually and do them one at a time and make sure it yields what you are looking for.
Best mod I made to both PRRI amps? Installing 14" Fender tilt back leg kits. Major improvement in overall tone and projection.