What tonearm will better mine?


I have been running a 12" Jelco SA750L with an upgraded Decca Super Gold on a Garrard 401 for a few years. I'm happy with the sound and happy with my pre, power and speakers. (Croft 25R, EICO HF35s, -good tubes, Quad 57).
I keep hearing that my tonearm is a weak link even though it sounds pretty awesome to me. What would it take to replace the Jelco and make a significant improvement? What should I expect to hear better? Would I have to drill the massive plinth? Thanks.
128x128noromance
Found this post and wanted to update it. Ended up upgrading a while back to a Jelco TK-850L 12". I covered my experience in another thread. Suffice to say, the 850 is in a different league than the 750.
@stringreen I don't care. They weren't criticizing, they were critiquing. Which is fine by me. Anyhow that was 2 years ago. I now know my rig sounds as good as many costing a lot more. 
noromance 
Glad to hear you made a tonearm change with a great result.

I bought an Audiosilente idler wheel based on your comments and it was a nice upgrade.

Thanks.

Jim Perry