What External DAC pairs best with Audio Research Equipment?

@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

I think a nice R2R DAC would increase the soundstage, the Denafips Terminator 15th or the Terminator 15th Plus (for a bit more money) would excel at that. The top end is also very detailed. It had avery positive review at THE Absolute Sound. 

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.