Originally, the two companies marketed together. This was in the era -early ’70s- when the first generation of "high end" equipment was being made. (I distinguish that era- which included JG Holt’s Stereophile, erratically published out of Elwyn, Pa. and the emergence of The Absolute Sound and Harry Pearson as two of the leading proponents of the subjective school of listening/evaluation, from what I label as the (earlier) golden era--Saul Marantz tube gear before he sold to Superscope, KLH 9s (double pairs), Quad 57s (double pairs eventually using Decca Ribbons and a Hartley woofer), old Bozak ConcertMasters, early Fisher and HH Scott tube gear, McIntosh tube gear, etc.)
I was an early proponent of ARC and owned their gear starting in 1975, after having heard and compared it to other high end equipment in 1973-75. The marriage of the two was, to some degree for marketing purposes--ARC was a Magnepan distributor in the early days. It was common to see the big Tympani (IU [?]), a very large multi-panel array demonstrated with ARC tube gear and often, ARC dealers carried Magnepan.
Both companies seem to have soldiered on through the years, despite considerable changes in the marketplace. ARC was one of the few to promote tube amplification in an era focused on high power solid state amps and specification battles over things like "transient intermodulation distortion" and "damping factor" and "slew rate" when the typical ARC demo consisted of playing a record and letting you hear what their gear could do. It usually impressed in its sheer musicality at the time when other systems, particularly solid state, had an electronic "glaze" and did not sound as convincingly real.
I have an old "sell sheet" from one of my first ARC systems, circa 1975, that included both ARC and Magnepan product listings and prices. I seem to recall bi- and tri-amplification but only remember the really big tube amps coming from ARC a bit later. Their earlier efforts, after total redo/mods of Dynaco equipment, were typically 60-75 watt stereo basic amps. (I’m working from memory here).
As mentioned, the market has changed considerably since then. There are a lot of different tube offerings, with different circuit configurations and "tube audio" became more than the tiny niche of hold outs and audio outlaws to become a recognized thing, along with greater power.
I stopped on the ARC path, not because of any problem or issue with the company (though the sound of the products changed over the years, from tubey warmth, to a period of "bleached" sound, to the era of the Reference series); I went in the direction of extremely high efficiency horns and low powered SETs.
Magnepans are power hogs. At least the bigger older ones were. And that means a pretty powerful tube amp. I’m not going to pretend to be current on either company’s offerings because I’m not. But I think I’ve given you a capsule history of their origins and early relationship having been there at the time.
PS: not to be a pedant, but Audio Research used to be referred to as ARC because the first company to offer the "acoustic suspension" speaker and the original suspension turntable, headed by Edgar Villchur, was known as "AR." Today, I’m not sure that brand is still active. Good luck and happy hunting, there are a lot of people on this board with experience in both ARC and Magneplanar.
ARC amps are designed with no fuses in the circuits, so when a tube blows (not uncommon in tube amps), it takes with it a "sacrificial" resistor. That means a trip to the dealer (or ARC), unless you are handy with a soldering gun.