Have you cleaned all the connections sometime in the last 50 years? :) Seriously, though, maybe cleaning all of the connections would help. Worn preamp tubes generally sound dull, rolled off highs and flabby bass- at least on my old ARC preamp.
CHecking tubes?
I'm bi-coastal, and twice a year I have to get used to the sound of a new system. I'm not liking what I hear through my Jolida 202, with ancient KLH speakers (only on choral music, which I realize stresses a lot of systems). 50 years ago, as step 1, I would have taken the tubes to Sears to have them tested, before starting swapping equipment out. Or should step 1 be ungrading speakers to something made less than a century ago? Thanks.