is there a way to tighten pins in tube sockets?

Yes. If you look at the sockets you'll see copper wires in socket grooves, bent over towards the outside of the socket. You can insert something behind this wire, between it and the socket wall, and bend it forward. It won't take much, just a tad. I use a small flat blade screwdriver (very small, like one for eyeglasses). BE SURE TO TURN OFF THEN UNPLUG THE AMP FIRST!!!!! :-)

FWIW.

Beyond unplugging the amp: be certain the power supply capacitors are FULLY DISCHARGED. Many components have bleed resistors, that will discharge them, after the unit is turned off(takes a little while). The caps can still somewhat recharge themselves, do to Dielectric Absorption. Whether your unit has bleed resistors or not: it would be best to leave the amp off/unplugged for a couple hours, then touch a short wire(insulated from you) or screwdriver across every capacitor's terminals, to insure that they are not retaining a charge.