Need to learn about Audio Research

I have owned one or more Audio Research components for over forty years. Never needing any adjustments or repair. Over time as I better understood what music really sounds like and more able to afford top notch equipment moved to all Audio Research. You can see my system under my user ID. Audio Research produces some of the best, most reliable, audiophile equipment in the business and has been a trusted name for fifty years. 

I used to own one of the ARC amps from what many consider the 'golden age' of ARC...the mid to late 80's. This amp was the ARC D70Mk2..and it was one of the best sounding amps I have ever heard. Today, this amp would still be competitive with some of the best on the market. BUT...and here's the thing, it was as noted above, not that easy to work on if it had an issue, and more importantly, it had a criminal protocol if one had to bias the tubes. ( which was required every so often!) 

Criminal as to the potential risk for electrocution! 

Today's ARC's are far superior ( but not necessarily in SQ), but they come at a cost.

I have an SP6 preamp and a D75 amp. That combination will work and sound excellent with Magneplanars. IMO it will have more of the tube "goodness" than later Audio Research gear.

My experience pretty much mirrors @whart  I well remember hearing the big Maggies with all ARC setups, one of my early references for high end sound, this in 1970's. I only owned a single ARC piece, this SP9MkIII, very disappointing, one of their 'bleached' out offerings.

I too migrated towards horns and SET amps, this gradual move over a number of decades and experience with a variety of amp topologies. I don't doubt you'd find some more recent ARC offerings to match well with the Maggies, my issue is the more powerful the tube amp the greater chance for reliability issues and expensive repairs due to added complexity. High power tube amps put out a lot of heat, heat is the enemy of reliability.

This is no way to buy audio equipment. If you're looking to fail and be unhappy with your system, this is a good way to do it. You have a pair of Magnepan 3.7's. Look at the recommendations you're getting. All are completely different and everyone knows they're right. Its very easy to go out and buy good components. The problem is you can buy all good components but the system can sound like crap because they don't work well together. The 3.7's are not forgiving speakers to work with. You can't rely on other peoples opinions. If you happen to get it right, it will be completely accidental. 

The only way to move forward that makes any sense is to go back to when you auditioned your speakers. What kind of electronics were they using, and what makes you want to buy something different? You need to have some type of reference before you start. And if this was an impulse purchase, you may want to cut your losses now and just get a speaker that's more forgiving to work with.