Personally I never liked the sound of the 70s/80s Japanese receivers, they had a very solid-state hardness to them. More importantly, to me, is that you do need to attend to the output impedance of the receiver you chose, as some have op amps, but I believe the majority have a 200-340 ohm series resistor hanging off the main amp out.
The high series resistor will almost certainly cause a rise of 1-3dB in the bass region of a phone with a high impedance, maybe even greater effect on those with low impedance, which will add a warmer color, if that's what you want. Before taking this path, it'd be a good idea to compare to what you're using or considering in dedicated phone gear to see if this fits your needs and tastes.
Personally, I tried to run my JH16s off the output of my Onkyo (320ohm, if I recall) and the effect was really awful and bloated sounding, to me. The 650's were a better match, but still sounded like they'd eaten a bit too much pasta and beer the night before....
This couldn't be further from the truth (my experience is the exact opposite). I'm specifically referring to integrated amps, but 70s/80s high-end receivers (which can be found for little money on the used market) were awesome sounding as well. Many of them sound better than most head amps under a grand. I don't want to go too far off-topic, so I'll just refer you to this thread: http://www.head-fi.org/forum/thread/537704/calling-all-vintage-integrated-receiver-owners
(I'm not denying your impressions of your own Onkyo though, I haven't heard that particular one).