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

ARC over Mac any day but if you want warmth try CJ.

@bobbydd - I’m with Mazian’s comment above. As you mentioned warmth, I recommend the Conrad Johnson Classic 120. That amp uses EL34 tubes and brings a wonderful warmth to the music. I alternate it with an Audio Hungary power amp. The preamp is a Linear Tube Audio Microzotl, which has a tube tone, wonderful soundstage and imaging, with some solid state detail. 
 

The above components, at least for me, have delivered what I was looking for - which it sounds like what you are also looking for - tube warmth with solid state details. Highly recommended combinations. 

Because the 3 little bones in your ear (ossicles) are different in every human what you hear is unique to you and as such you must have an in-home test of both amps to get your ears happy.

That being said, (I'm biased) the ARC Ref 75SE is a fantastic amp that will play well with your Ref 2,  It's a neutrally voiced amp that adds very little tonal expression to the signal you feed it. The amp has a wispy airy soundstage and has well controlled bass. It's faithful to the artists recording, and to my ears well recorded music sounds live and deep through ARC gear. 

The Mc is altogether different. It's voiced to be warm and "Tubey" sounding which is great for some tracks but in my biased opinion is adding too much to the original signal. It's pretty, even sexy. It has good resale value. It just colors outside the lines too much for me and it's irritating to have a track that's supposed to be bright and sparkly sound blanketed. 

I know this will tick off some people but the Mc is considered a "Mid Fi" product. 

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.