Last high-end 2-channel audio development of Onkyo, launched at the same time as the Japan-only and diminutive A-1E (and C-1E companion).
The M-588 embodies the refinement of what was put inside the M-508 (1986) but gone were the In-Phase transformers...
The input stage is a double differential followed by a symmetrical cascode with mirrored current regulations. The schematic below shows the double-differential and double servo.
The low-impedance driver stages are located on the same boards as the output stages' twelve 2SA1303 / 2SC3284 Sanken transistors which all have a Pc of 125 !
PCB tracks are thick 70µ, componentry is select, 2-gang input volume pots, Neutrik input terminals, thick internal cabling, oversized rectifying diodes, eight 5200µF / 100V low-ESR caps (I belie-ve custom ELNAs), and the two very big transformers are of the EI "laser" kind (faster response, linear flux).
Also present is a (very silent) front headphones' output which cuts the main terminals when used.
Protection kicks in by way of opto-coupled relays. But protection never kicks in : the M-588 is a true powerhouse, stable down to 2 Ohm and with square wave skewing of only 11% at 40Hz and... 0% at 1Khz !
The M-588 and P-388 were however also exported outside Japan and, depending on country, available until the very late 1990s.
The original models are champagne in colour (M-588), the general export models were (sadly) silver (M-588F) with plasticky sidburns while the german model was full black with plasticky black sideburns (M-5890).