Jackyjack,
No, you really can't overcome a fundamental compliance mismatch between cartridge and tonearm. Adding mass will simply add mass where neither the tonearm or cartridge designer wanted it, and still won't overcome the basic mismatch problem.
You can order a Denon DL-110 HOMC cartridge (plays through the MM stage) for just $87. I would give that a try first and see if it sounds better than your DL-103R before dropping some major gelt on a (possibly unnecessary) external phono amp.
If you think that you need a new phono amp on a budget, I have had heard good results with the Lehmann line. They have units in all price ranges from <$500 to approx. $2K.
Having said that, there are lots of guys who use and enjoy the sound of their DL-103 cartridges on lighter-weight tonearms. I'm just not one of them.