If you're asking what single upgrade would yield the most cost-effective benefit to SQ, I'd suggest a new cartridge. Another bang-for-bucks upgrade would be to get an LPS to replace the wall wart on your phono preamp. MS sells one that would be compatible with any other pre in their line-up. You could also go (much) cheaper with a similar voltage after-market LPS, but as with all things audio, you may not get as much improvement.