Very successful combo with a bit more detailed attention to looks than its previous siblings (P-306 / M-306 etc) and some circuit nifties to do between preamp and amp what others were doing between amp and loudspeakers.
The M-509 naturally is the W Servo receiver for "eliminating DC and subsonic problems and differences in ground potential ; W Super Servo is equivalent to 100-times larger power-supply."
DC structure with a custom sliding bias system which avoided the constant switchings of Class B and the continuously sliding bias (in Class A) of most of the contemporary systems.
The check point of safe operation is at the pre-driver, detecting temp differences between each output transistor, bias stability and stable feedback loop - that's Servo Technology.
Four 30,000µF / 90V caps, sextuple (!) push-pull of Sanken 2SC2773 / 2SA1169 output transistors, sealed relays and, again, non-magnetic metals. 31kg in toto !
At 600,000¥, the P-309 / M-509 combo was clearly on the high-end side of (somewhat) affordable audiophilia ; it nevertheless sold very well, in Japan.
The next step was the Grand Integra M-510 monster which was however not graced with a dedicated preamp and didn't use the W Servo technology ; the ultimate step was the 1992 M-588.
Thank you Roman Schütz for having reminded me of two points :
1. The M-509 was an M-5090 in germany.
2. The M-5090 were also later on in Germany released under the M-200 name with different scripting and Integra badge added.
This was probably a way for Onkyo to get rid of its japanese stocks as the xx9 combo, in 1985/86, already was old salad, especially the P-309.
Nudies of an M-509 at the indispensable amp8.com : here and here.