What should I upgrade first on my budget system?

The rest of the OP's system is fine! Depending upon how much he has invested in LP's a cartridge upgrade would be good. A Hana SL plus an SUT is an easy recommendation. Keep the PS Audio preamp and the Will Vincent Baldwin tube amp. Both units offer excellent sound quality! 

@ghdprentice : I think you are mistaken when you state that streamer sound quality is better than CD. You are at the mercy of whatever compression schemes the streaming services use to squeeze their data files to fit on the digital pipelines! No thanks! I will stick with my CD players/transports and extensive CD collection. 

For a DAC I can recommend the Khadas Tone Pro 2 for around $249. Keep the Rotel in use as a transport. I have the less-costly Khadas KTB. Both are competitive with any DAC below $1K.

@jasonbourne52 Your mention of having DCM Time Windows and the Ohm made my day. I use to sell them way back when. Great speakers. The Time Windows blew my mind. Haven't heard them in too many years to count, but they would give many speakers today a run for their money. Thanks for the memory.

no you guys are all wrong, a great system is built around the source, without that nothing else matters.