What should I upgrade first on my budget system?

A system is built around a good speaker pair.  I 'm not familiar with your speakers.  Make sure you're happy with them for the long term.  A few more dB would be nice for your tube amp.

That amp will probaly do you well for a while.

I'd then look for the best dAC you can afford.

Jerry

Perhaps some room treatments is where I would look, first. 

Have you looked into other turntable cartridges?

I think your speakers are probably the best component in your system. Typically you start there. But in your case I think you may wait until last. 

I always try and get a big improvement with each new component. 
 

I try and build all components to a new and higher level. Never investing less than double… hopefully more. 

I would probably look for a preamp first… it is central and is so important of setting the tone. Then amp. But one could argue you may get more bang for the buck with the amp first (pun intended). 
 

Next a DAC… which will elevate your digital play… at this point you may use a laptop or other source as a streamer until you can afford a stand alone streamer. Streaming today can easily exceed your CD player in sound quality and replace buying CDs, allowing nearly infinite music for $14.99 per month (Qobuz). 
 

Then you would be ready to upgrade your speakers.

I would look for used audiophile equipment less than say 15 years old. Especially for the preamp and amp.