Bimmer…in my experience the best ROI in interconnects is preamp to amp. There are exceptions though. In case of silver/ copper mix, having silver between source and pre and copper from pre to amp always sounded more natural to me. In example I had AZ Silver Ref II and Matrix Ref II interconnects. The Matrix between pre and amp were much better the Silver between there. And Matrix is a cheaper cable. You have to try.
If I were to upgrade cables, I’d start with speaker cables though and work my way up the audio chain.
Have you considered cartridge and phono amp upgrade?