What should I upgrade first on my budget system?

The right Cd/amp power cable upgrade can be as good as new component.

@digsmithd +1

I would not call it a budget system.

Biggest bang for the buck? The turntable?

I know nothing about your speakers but they look very serious. Not the best match to your amp though, depending on your space/room size  (very broad generalization: if you are in Montana, your room may be big :) )

Room treatment, power conditioner, OCC interconnects and speaker cable, isolation footers, etc.

Good/right footers on my sim player made huge difference...good call dweller.