"If it ain't broke don't fix it." I own and have owned many pieces of vintage equipment 40+ years old that I start on a variac and they sound terrific without ever having any maintenance besides tubes.
Maintenance for Tube Preamps
I've enjoyed my Conrad Johnson tube preamp for a decade or so. Other than replacing the tubes a few times over the years there hasn't been anything else done to it. Are there other maintenance tasks that I should attend to? I believe all the capacitors are polypropylene (no pesky electrolytics!). All seems well, but I want to make sure I'm treating it properly.