Pioneer M-90
Pioneer M-90a
Pioneer M-91
(1986 - 1988 - 1997)

Companions to the C-90, C-90a and C-91 preamplifiers but with much less visible differences between them.
However, the original M-90 and the subsequent M-90a/M-91 are built quite differently, although they look... identical.

The M-90 is a 20kg beast with two 220VA EI transformers, 4 parallel push-pull pairs of 130W Pc transistors and 48,000µF of smoothing caps - the japanese M-90a seems to differ on this with only 40,000µF. Caps are aligned just behind the front plate ; the two trafos make a straight line toward the back plate with the heatsinks and power boards flanking the latter on the sides ; total current capability is of 47A.
See an M-90 in repair, part 1 and part 2

The M-90a has the same basic structure but swaps the EI transformers with the cast-iron ones you can see below and the thin fin heatsinks with beefier honeycombed ones ; caps are maintained at 48,000µF. Also a thick two-part top plate is added, 1,6mm honeycomb bottom chassis, resin-cast capacitors, five cell-polymer-filled insulators - and more to mainly strengthen mechanical stability (going from 20kg to 28kg !) and heat dissipation.
The 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.

Specs are otherwise identical and common features are Non-Switching structure, 70µ PCB tracks, input/output comparators, very large and very flashy red meters and 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 4Ohm...6Ohm position being restricted to using only one pair of loudspeakers - the M-90/90a 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, along the M-72 and C-72 combo.