Which component to upgrade 1st ?

I would strongly consider replacing the speakers first.

That’s an interesting thought. What would you suggest?

Well, first, you are well on your way to a really great system. Everything you have said suggests you are on the right track.

The Audio Research REF5 is a fantastic preamp. I owned a REF5SE for ten years and now own a REF6se. I think this should be your first step. It will impact everything. If you are not familiar with my systems, look under my UserID. the more ARC… each step will make a big difference. Your budget is good to make a significant step up. Your ARC REF75 is a great classic amp.

I would say, the next move would be the streamer. I would recommend a used Aurender N10. You could get one for around that cost. After owning lots of streamers, my conclusion is that the investment should be at the level of the rest of your components… my amp and streamer are the most expensive electronic components I have, and really well worth it. I currently own an Aurender N100 and their flagship WE20se.

It depends on what you listen to the most ; digital or analog. If you predominantly listen to digital put your money there and the same goes for analog. 

Interesting!

I wonder how a streamer can make your ears hurt.