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.
So the P-309 has two pairs of outputs : a standard one (output 2) and one that is doubled between signal (L/R) and "servo sensor (L/R)". You guessed it : a special cable was to link the P-309 and M-509 to avoid resistance, capacitance or inductive elements added to the signal and make believe that preamp and amp were actually one and not two, distant, entities.
This was called W Servo Technology but however only did work with the M-509 with two dedicated Y cables with grounding cable.
This system was first used by Onkyo in the P-306 / M-506 (1978), P-306R / M-506R (1981), P-306RS / M-506RS (1983) and P-307 / M-507 (1978) - the M-507 being a very close version of the M-509.
The rest of the P-309 was just as seriously built :
Class A and DC everywhere, passive components for the tone controls, copper busses and shielding, regulations aplenty, non-magnetic metals, high Gm FET differential amplifiers for the relay-switched inputs (in a single aluminiumm casing) and low-impedance circuit and output.
The EQ phono stage was designed so that it would function like a power-amplifier, able to output the equivalent of a 5W signal !
Thank you Roman Schütz for having reminded me of two points :
1. The P-509 was an P-5090 in germany.
2. The P-5090 were also later on in Germany released under the P-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.