OP here; thanks for all the input and hearty debate.
I've decided that I'll start with the cartridge. I'm happy with the speakers and amp, and believe they're each doing everything they can with what they've been provided. Therefore, I must upgrade the quality of the signal the speakers and amp are getting.
No, @ditusa, not a Bot. But I appreciate the motivation to get me to respond back and close the loop.
Cheers!