Very well remembered Mozilla-esque & Gargantua-esque heavyweight powerhouse which needs two people to be lifted : 63 kilos.

Able to output 2x 1,2kW at 1 Ohm, one might imagine this having been aimed at PA reinforcement for deaf audiences but, in fact, the circuit's engineering are very much fine-tuned and almost microscopic.

The secret inside lies in the two secondary transformers : these are "In Phase" transformers which echo the Signal In-Phase Filters of the Integra P-308 high-end preamplifier. In Phase transformers are to cope with the reactive load of loudspeakers, and not only their resistive aspect ; these transformers are to avoid the phase shift between voltage and current in the amp-to-speaker signal path.
Most pronounced around the speaker's resonance frequency, this phase-shift also happens between the voltage and the charging current in the amplifier's power supply ; these charging currents may start fluctuating along the low-frequencies contained in the musical signal. Out-of-phase charging currents can generate electromagnetic flux which in turn often induces voltages of the same incorrect phase in the nearby driver stage (through which the audio signal passes) ; the problem is then, naturally, sent to the loudspeaker... muddy bass and blurry soundstage.
Real Phase transformer are inserted between the positive and negative charging currents of the the power-supply and the capacitors : as the positive and negative currents pass through the transformer's two windings, unwanted peaks and dips cancel each other out - in phase ! You can see the schematic of the Real Phase system by clicking the "more" button below - click !

Otherwise, the Grand Integra M-510 is real dual-mono amplifier : two power cords and two toroidal power transformers ! A fifth and common trafo is used for the meters and out-of-the-signal-path circuitry such as speaker relays. Each channel benefits from two 33,000µF / 100V capacitors - for a 99,000µF total... Each channel's power board holds 7 pairs of (I believe Sanken) transistors, 28 in toto. Gargantua-esque indeed.
Linear Phase Switching is included for true waveform linearity (see the Integra P-308 page on TVK for details) and an elaborate complex protection circuitry kicks in if necessary - 1,2kW per channel can transform your fragile tweeters into live frisbees.

A cool "waiting monitor" flashes some leds geometrically while a dimmer allows to switch off the VU meters' lamps ; gold-plated terminals and solid wood sideburns give the ultimate in reliability and looks, respectively. Both the variable and direct inputs are single-ended, though. The front flap hides the speaker selector (A, B, A+B), variable input pots, dimmer, and meter range selector (x1 or x0,1).
The Onkyo Grand Integra M-510 was lavishly displayed in a "Big New Sound" section in Stereo Sound #73 (winter 1985), and was seen in many a japanese Onkyo ad, often accompanied by the Grand Scepter GS-1 horn loudspeaker. However, this golden Grand Integra now remains... quite invisible.

Looks-wise a bit heavy-handed and tacky and not really discrete with its 27cm in height, but, hey - you can't be Gargantua and look like Minnie the Mouse !