It looks like a V-FET amp... but it ain't.
Housed in the chassis of the TAN-8550 allright, but sans V-FET : inside are six pairs of 2SB520 / 2SD340 bipolars.
Housed in the chassis of the TAN-8550 allright, but with a power limiter : the TAN-8250 can become a 500W monoblock !
The TAN-8250 is a phase-linear dc pure-complementary circuit biased in Class A operation for everything but the power board.
Mirko Essling noted similarities inherited from the old TA-3200F bestseller - more about that later.
The output transistors are hooked to the double heatsinks like the V-FETs amplifiers (from below) and the diecast heatsinks are doubled with heat "chimneys" for even better convection.
The transformer is enclosed in a square box, no doubt to conceal its toroidal nature - we're in Sony territory so let's not show the good stuff :)
Features-wise, the 8250 bears more front switches than its TAN-8550 V-FET counterpart because of the three-step power-limiter.
The rest, front and back is equivalent : speaker B level control, DC or not DC inputs with front selector and individual level pots, three 'speakers terminals, plus meter sensitivity switch.
The speaker B control is the adjust output of lower-sensitivity speakers to that of the main (A) speakers. The third 'speaker output is called DIRECT and these are directly connected to the amplifiers' output point to provide a very high damping factor.
The inside of the 8250 is packed with said transformer (placed vertically in its square box), one choke coil box and a dual 10,000uF /70V capacitor (for a 20,000uF total) and the (invivible but from under) dual rails of diecast heatsinks.
The rest of the space is occupied by the Peak Program Meters box - big box.
Peak Program Meters rely on two independent mirrors to display peak and VU modes in a manner most intelligible - and beautiful. If Sony were to release the PPM meters today as pro studio accessories, they would sell like hotcakes.
These Peak Program Meters were included only a handful of units because of their size and manufacturing costs : TAE-8450 preamp, TAN-8550 amplifier, TC-880-2 open-reel recorder and... that's it.
The TAN-8250 is however a very very rare component : Sony probably should've kept the TAN-8250 as a V-FET but leaving the expensive PPM out.
It would have made more sense that way, marketing-wise, than to have a visually identical twin based, inside, on a completely different amplifiying technology...
Saving on the V-FETs didn't boost sales, though, and, at 1300$ in 1975, one can easily understand why.