The difference is in the multiplex unit. Unlike the MR-66 which had an outboard MPX unit, the MR-65 multiplex had an internal adaptor.
The early version of the MR-65 was a monaural tuner that included a stereo mpx input on the tuner chassis; which the MA-5 stereo multiplexer attached to.
In the later MR65B, this unit was already installed at the factory, and while virtually identical to the MA-5, called the MX-1.
After the MR-65B was introduced to the market, the MR-65A became the designation for the mono version of this tuner, to distinguish it from the factory stereo version.
I have two beautiful MR-65B's and think they are among the best sounding tuners that I have ever owned.
There is much hype over the later MR-67 and MR-71, but I seriously doubt that there can be much of an improvement in sound quality, given the similar circuit topology -- especially considering the nearly identical tube complement of the MR-65B and MR-71. I expect that the differences between the two has more to do with DX'ing capablities than actual sound quality.
I would imagine that all of McIntosh's tuber tuners sound excellent when properly aligned, be they mono or stereo.
Regards,
Jimmyblues