SQ=the only important thing to me.
which would be the best value?
what brands\models of old receivers to look for?
Well the really good receivers of the bygone era are really expensiver like the Mcintosh around $500-1500 and then again you could buy a decent combo if DAc and amp for that price too. Only way to know is to try. Some people say lowend headphone amps like LittleDot MkIV pale in comparison to the Mcintosh, but then again the Mcintosh is pretty darn expensive.
Nikko has the best value on the used market!
I can't believe nobody has mentioned any of the vintage pioneer receivers from the mid to late 70's quite possibly some of the best depending on what model you get and for what price you acquire it for.
I might be biased though since I use a pioneer sx-939 as my main amp for cans, and it sounds fantastic.