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
Last night, I added the Ammonite collar and the Expressimo Audio brass stub and counterweight to a black 12" Jelco 750L (401 on slate plinth). Both are extremely well made. I did the mods one at a time and the most impact was obtained from the brass stub/CW. Scott Hamilton - Is a Good Wind Who Is Blowing Us No III -increased speed and air on the edge of brush drums. More blackness etc. Not huge but enough to be worth it and it looks awesome. The collar didn't make as big a difference as far as I could ascertain but I may have messed up the test by running on cold tube amps. All in all worthwhile and easy upgrades. 
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