I've had the exact same problem as you with a modded Rega arm I installed on a Thorens TD160. I think shims are a bad idea, compromise a tight headshell, cartridge interface.
In my case I had to cut down the height of my armboard in order to get the arm lower than level with the vinyl. I'm not sure you can do this with the Rega tt. You can also try the Herbie's record mats, they have a thicker one which will act to lower your arm.
I'm also using the Denon 103R in this setup, sounds nice to me. I will add increased mass to my arm at some point in order to improve tracking on this low compliance cartridge.
In my case I had to cut down the height of my armboard in order to get the arm lower than level with the vinyl. I'm not sure you can do this with the Rega tt. You can also try the Herbie's record mats, they have a thicker one which will act to lower your arm.
I'm also using the Denon 103R in this setup, sounds nice to me. I will add increased mass to my arm at some point in order to improve tracking on this low compliance cartridge.