Transitional version before that segment of the original ESOTEC line would be replaced with the PM-84 and PM-94.
The PM-6a is a Pm-6 with the new Quarter-A "sliding Bias" system to automatically step from Class A to Class B according to the power to be output - please refer to the PM-84 page on TVK for the details of Quarter A.
The caps assembly and transformer would be transferred "as is" into the PM-84.
Not many differences beyond this, except a much becoming lifting in looks (the influence of Marantz Japan, for sure), swapping the matte knobs with shiny ones.
Also, by 1983, the Marantz ESOTEC tag had definitely vanished from the landscape.
The Pm-6a probably is the classiest of all Esotecs, with a wood enclosure unfortunately available in Japan only.
Nudies of a real 6a at the indispensable amp8.com.