What External DAC pairs best with Audio Research Equipment?

There is no “pairs best” — there’s only what specific improvements you are looking for and what sound characteristics are most important to you.  If you elaborate more on that you’ll get much more meaningful and helpful recommendations here, otherwise you’re just getting recommendations for what other people like, which may have little in common with what you’re looking for. 

Soix great point…I currently have a very warm sound and would like to increase both the soundstage (meaning more holographic) and the detail accuracy of the instruments..if that makes any sense?thank u for making that point 

@2andy since you have ARC amplification chain and you have ARC DAC9 at the top of the list, it feels like that would be your preference. 
Synergy with all the ARC gear removes the headache of voltage and impedance matching. Plus you get consistent sonic signature that you don’t need to guess how to balance out bright or too warm. 
Only thing I would check if buying a used DAC 9 is the USB card compatibility with your streamer. I believe the earlier models were not compatible with Linux based streamers. ARC should be able to tell you if you provide them the serial number. 

Goos luck!

DAC 9.

Arc gear colors the signal pretty heavily and I’ve never had a piece of gear that didn’t sound good through it. So what I’m saying is you don’t need something to “match” with it. If anything you can add a dac 9 to a non arc system to get the arc sound.  But arc has never been known to have that great of a dac.  That being said, the most 3dimensionality, height like a projector above the speakers and most timbral color system I ever heard was arc with dcs apex gear. That is my end goal 

but I’ve had dacs like laiv Harmony, extremely 3 dimensional for cheap dac but very smooth, denafrips terminator and plus, great everything, Meitner and esoteric. They all sound wonderful through arc gear and are known for their dacs