I have been using spray cleaner that leaves no residue. My tube sockets
are gold plated, which seem to work nicely over the years.
I tried an experiment, some year or more ago, of fastening the speaker
wires to the speaker jacks as an anchor, but soldering a wire directly
from the speaker wires to the crossover, eliminating one physical pressure connection.
It is working fine as I found the bass was more consistent. Maybe my fingers are weak from age, but I found that tightening with a wrench improved the bass tautness, by increasing contact area due to surface irregularities, and is gas tight. (Of course I did not strip the threads.)
Might be something worth trying.