Rega upgrades/tweeks

There may be some Expressimo's left (they're out of biz), but the state of the art is considered by many to be the Michell TecnoWeight. I'll let others describe it with audiophile language, but it seems to extend bass, help tracking, and give you accurate (.1g) TFA. Forget name, but look for on Ebay and you'll get new for around $125.

Check out the Kerry Audio Design ad here on Audiogon. This topic has been covered many times, but ancient discussions may have been deleted, by now.
My own experience with the Kerry Audio Design F-2 Titanium H/W Counterweight is that there are across-the-board improvements in performance of an RB-300 just waiting to be unveiled.
Not sure why one can't mess with the Rega sound (although that turn of phrase does make it sound dangerous to try) especially since so many competing counterweights seem to please Rega fans.
Gregory Kerry's design delivers better bass weight and texture, solidity of imaging, harmonic richness, nimble transient handling, and on, and on. And, his ad refers to similar praise from audio press experts.

i am a previous owner of a p25 and found the heavy weight and the collar for adjusting vta helpful and beneficial for the overall sound. i have both available as well as a ringmat, which also helped. i have since upgraded a few times. let me know if interested. doug krieg.

If the mass of two counterweights is the same, how can one "sound" any different than the other?

There is a Bronze weight from Germany I think,that states as part of their advertising,that the Bronze metal works the best on the Rega arm and that is why their weight is superior.It would have something to do with the way it dissappated spurious vibration I would imagine.