The Pioneer SX650 utilizes thick-film-hybrid output devices. They are early integrated circuits, the Sanken SP-40W. These outputs contain the final stage as well as the bias stage.
Do not run 4 ohm speaker loads with an sx650 if you're heavy-handed with the tone controls/loudness, or of you've got something atop the unit.
That being said, I owned no less than 3 of these, and they're a definite 'sweet-spot' in the pioneer sx-x50 line!
During my tenure with the 650 as my main stereo, I ran Celestion speakers and sennheiser HD555 cans off it. very nice sonic presentation IMO.
I recently had a mint-with-manuals-and-warranty-card Pioneer SX-1280 taken away from me in the hurricane. I couldn't care less, though since despite it's beauty and power meters and high output, it lacked finesse in the 'nth-degree' . It was more of a brute, and nothing like an sx-1250 in terms of sonics.
Hell of a fun unit though!
Favorites of mine in the vintage SS arena include:
Sansui 4000 receiver, 45wpc
Sansui 3000A receiver (watch out for hack mods and unrestored ones since they can set a woofer VC on fire) 48wpc
Kenwood KA-8100 integrated amplifier 75wpc
Fisher 500T receiver ~40wpc early silicon and germanium
Akai AM-2800 integrated 80wpc
Yamaha CR-2020 receiver 125wpc
Luxman R-1070 receiver 70wpc (Lux gear exudes quality, even to this day with their current offerings)