What External DAC pairs best with Audio Research Equipment?

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.

staying in house always gives some assurance of good matching of gear and tonality but beware that companies such as arc have been around for many many years and their gear from different decades can sound somewhat different and mate as differently as different makes of gear

second point is in my experience if you have a a fairly modern (2000 and forward) arc pre/line stage it will take the input of any modern dac just fine -- so go for what sonic flavor of dac you see fit to suit your personal musical palette

The DAC 9 is almost 10 years old and DACs have improved a lot since then. I also don't think DAC/linestage matching is particularly difficult, so my approach is/was to get the best DAC I felt would be the best fit for me in its own right, within my budget.

I have a Mola Mola Tambaqui feeding into an ARC Ref 10 linestage. It's a pretty fantastic combination.

I’ve A/b’ed myTopping d90III Sabre dac against the Hegel.   It compares very favorably and is more towards the detail goal stated with similar excellent soundstage so that might be worth a try and save some $$$s to boot. You can hear every detail in a recording including incredible micro detail with instruments in good quality recordings.