Hi Mr. Eggontoast!
"I have a vintage Pioneer SA-7800 from 1978 and it sounds great".
What?
I'm surprised! It seems you're not that fussy audiophile who I thought you were ...
Pioneer SA-7800 has never been an audiophile piece, even in late 70's. I agree on the basis there are amplifiers which still today can be highly regarded in terms of audio quality, but your amp isn't one of them certainly. By the way, SA-7800 was retailed at 50.000 yens in 1978, within a price range of budget equipment.
I used to have a SA7800 when living in Japan. For my liking, it sounds veiled on mids and bass lines are a bit boomy. It's easily outperformed by modern entry level models of brands like NAD, Rotel or Cambridge Audio.
Nothing personal!
Regards,