Which ARC reference component to upgrade?

I had a Ref 5 and Ref 110 with Wilson Sashas and first upgraded to Ref 40 which was a big step and then from 110 to 150 which was close to the same jump in performance. I'd say in your case the Ref 5se upgrade may be slightly more pronounced since you are going from the Ref 3.

How about room threatment? That might be the best advice towards better sound.

Hi, have you tried driving you Ref 110 direct from your Dac , eliminate the line stage. I owned a Ref 110 and found it to be a bit overated. The Ref 110 only sings with easy loads as I think the output transformers are too small and easilly saturate. The result for me was the bass , although it would go deep it was some what slow and loose. Selecting the 4 ohm tap improved matters in the bottom end ,but dynamics then suffered. selecting the the 8 ohm tap restored dynamics at the loss in Bass definition. Please don't get me wrong , the Ref 110 was musical but didn't rock my boat. Your speakers dip below 4 ohms at 220 hz , see Stereophie measurements of your speakers. I believe the Ref 150 improved matters. My amplificatin is now VAC , this gave me what I was looking for as my speaker dip to 3 ohms in the bass. I don't expect you to buy VAC as I did although you could try it for yourself, the Ref 150 could be a good place to start, only my opinion.

Audio Research thrives on customers like this. That's what keeps them in business. I'm not a hater I also own ARC gear.