What External DAC pairs best with Audio Research Equipment?

Based on your response to soix, it sounds to me like you are not seeking more warmth for an already warm system. I have not heard any of the DAC’s listed, but from what I know, the Weiss would likely bring some sparkle and detail to your system.
 

My speakers are pretty laid back and I was running them with a warmer Merason DAC and I liked the sound. But I recently introduced a Bricasti M3 to the system and am very much enjoying the more incisive presentation. 

If you are using tube preamp and amp , it’s worth adding a solid state DAC . My choice is the new schiit Biggy. Audition it return it if you don’t like it. It might synergize with your system. My guess it will work. That’s what I did my schiit gungnir Raysonic cdp as transport , audio research preamp and mono block tube amp and it took away softnesss and added good bass and transient.I am amazed.

Audio is full of surprises , you will never know till you try it. Most of my success mostly is when I least expected the gear will work on my system.You have to invest time trying yourself. Agoners can guide you at the end it’s  what you like.

I use a Ref 5SE in my system with a Bryston BDA3 DAC. It sounds fantastic.

While I am sure the AR DAC9 would be a great match for your AR system (and look so good), I agree with those that have recommended a more resolving DAC.  I think another warm sound is too much of a (good?) thing.  I think the amazing DAC now had for a value used is the Mola Mola Tambaqui.  Great resolution and soundstage, world acclaimed.  Can be had now for under 8k.