bturk667
It sounds like you are switching to a heavier counterweight, which would balance the arm when set closer to the pivot compared to the stock arm counterweight. If so, this is indeed a way to reduce the tonearm system effective tip mass. (Think balance = sum(weight*distance) and inertia = sum(weight*distance*distance/Gc).

Reducing effective tonearm tipmass is difficult while increasing effective tipmass is rather easy.... simply add weight to the headshell and move the counterweight further from the pivot. In short, get thee an SME III improved fixed headshell to replace your Rega and then tune it to your cartridge. I would not waste money on expensive but ineffectual arm tweaks unless you have money to burn... and if you do have money to burn you can send it to my favorite charity.