Audio Research versus McIntosh tube amps: anyone experience or compare both?

The 75SE is one of the sweetest ARC amps. Detailed, superb soundstage, excellent instrument timbre, and easy to check and adjust bias. Agree it will pair perfectly with your Ref2MkII.  I did not have much success pairing Mac amp with ARC preamp despite "adequate" impedence matching. 

Ever thought about biamping. Maybe see if pairing your existing solid state monos and a 275 for McIntosh or better from audio research since you have their preamp for the mid range and tweeters. You have a nice size room and maybe it's underpowered. Lots of windows which you know about and maybe it's fine. I am in love with your rack.

I'm using a McIntosh MC 901 and find it works out really well . I'm exploring replacing all of the smaller tubes with NOS telefunkens. Replaced a few of the small tubes with good results and I need to complete the transition. I like the independent control of the drivers and I didn't want to deal with complexities of crossover components so that's why I got the 901. 

The ARC has often been referred to as a reference level component. No doubt  it's of the highest quality. Mcintosh is also used by many reviewers and audiophiles as a reference. Both companies (and others) have historically produced the best of the best. I think you have a fun time ahead ! Judge only with your ears and your system. Which one gives you what it is you think you're missing ? Oh how I envy your days ahead. Nice cabin view between those Joseph's.

  "Just one more thing" to the Pirate . . . McIntosh,  mid-fi ? Thanks for the chuckle!

I don't like it when people make a post that doesn't really answer the question asked, and I will start by admitting no McIntosh experience. I am, however, recently back and forth between a Ref 75 and the XA-25, and I actually find the XA to have more midrange warmth than the Ref 75. I really like them both a lot - Ref 75 edges out the Pass on soundstage, and Pass seems just a bit warmer/fuller in the midrange.

At least to my ears , if you are looking for more warmth in an amplifier as you suggest - I don't think that the Ref 75 will scratch that itch over the XA-25.

I at one point I had both a Ref 75SE and Pass xa25 in my system. I concur with @chcook's statement " I actually find the XA to have more midrange warmth than the Ref 75. I really like them both a lot - Ref 75 edges out the Pass on soundstage, and Pass seems just a bit warmer/fuller in the midrange." I will add that I thought the Ref75SE had a little blacker background.