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
@jperry 
Thanks Jim. Yes. I've since changed the AS idler for an Artisan Fidelity wheel. Not cheap at $400 inc. shipping and not as dramatic an improvement as the AS over the rubber original. Nevertheless, it tuned and brought more air to the bass and increased dimensionality and top end inner detail. 

@tzh21y
 Yes. The Jelco is quite good. Have you heard the 850?
@tzh21y As I said above, the 850 is a massive improvement on the 750. If interested, George Merrill has the best price.