I’ve bought nearly 20 used amps of various vintage, many 20-30 years old by now. Only two have had sound problems (as opposed to needing new bulbs in meters and that sort of thing): a Levinson 23 which are known to cook themselves to death, and a Classe that still works but has occasional right channel fussiness.
The rest work great.
IMHO, buying new vs used says more about the personality of the owner than the reliability of the gear (not saying either is superior). I have friends that just won’t own used.