I bought a 1989 SOTA TT with vacuum/Sumiko MMT arm/Dennon 103R retyped with elliptical/sapphire cantilever. ... What is the best way to get a noticeable improvement in my system without breaking the bank...
Change your cartridge.
Sumiko is nothing special, Denon designed for SUPER HEAVY TONEARMS.
Start with a new cartridge if you want to improve the sound, this is where the difference can be huge!