Rega P5 w/Denon 103R what should I upgrade to?

TS0711 I will respectfully demur. I have owned 8 or 9 Rega arms over the years, have added mass and the TWL counterweight as well as the Expressimo and various other geegaws and your idea of adding mass at the headshell and the counterweight will absolutely increase the effective mass of the arm.

It will also change the bending modes within the arm and hence the resonant structure of the arm as well as the load on the bearings. YMMV and appartently does.

No doubt, some will find this a solution, but I did not find it a substitute for a true high mass arm.

Hey like the comments maybe I should have given more info to you guys.
I have modded the J9 (6moons) except for the oamps which are on the way. I'm using NOS RCA tubes. My table is also modded counterweight, subplattter and dual white belts and a herbies mat. Thanks again for the info I appreciate your knowledge and learnings.

Vince, take the plunge on the Zu 103, it beats the stock 103, and is well worth the money ($800+). I've gone on to get the Expert Stylus And Cartridge Company here in the UK to mod it with Paratrace stylus and White Sapphire cantilever for £280 ($450+). It comfortably outpaces my 4x pricier Transfiguration Orpheus. An unbelievable giant killer that I would not swap out.
I have a thread devoted to the Zu 103 ESCCo mods.

Mingles-
I sub'd the stock tubes with cyro'd Mullard reissue 12AX7 with long plates. I also performed the rest of the upgrades on 6moons article.