preliminary
Companions to the C-90a and C-91 preamplifiers, the original M-90 and the subsequent M-90a/M-91 were built quite differently, although they do look... identical.
The M-91 is the USA name, under the "Elite" tag and M-90a is the EU/JP name.
The M-90a has the same basic structure as the original M-90 but swaps the EI transformers with the cast-iron trafos and the thin-fin heatsinks with beefier diecasts honeycombs ; caps are maintained at 48,000µF.
Also a thick two-part top plate was added, 1,6mm honeycomb bottom chassis, resin-cast caps, five cell-polymer-filled insulators - and more to mainly strengthen mechanical stability and heat dissipation : going from 20kg to 27,3kg !
The original EI transformers were however seemingly kept for the non-JP and non-US versions ; and the added top plate seemingly was for the units with potted transformers only...
European models weigh 23kg and therefore only saw half of the upgrades between the original M-90 and the M-90a / M-91.
Specs and features are otherwise identical : Non-Switching structure, 70µ PCB tracks, input/output comparators and very large and very flashy red meters.
Three inputs available : CD Direct, Line Direct and Control amp.
The first two bypass the input stages and buffers but go through the front volume pot ; said volume and switches in fact use a long shaft and relays, respectively, to keep signal paths as short as possible.
The small front flap hides speaker pair selectors (A/B), a welcome display on/off switch and a headphones plug.
A two-position switch at the back allows to select speaker's impedance, the 4 Ohm...6 Ohm position being restricted to using only one pair of loudspeakers - the M-91 is a high quality powerhouse but it isn't an MX-10000 !
Although the C-91 was replaced in 1993 by more obviously home-theater oriented receivers, the M-91 was kept available in the USA until 1997, alongwith the M-72 and C-72 combo.