I would stay away from Cramolin. It does tend to keep cleaning even when you don't want it to and can leave crud behind. I use pro gold but again if it isn't broke don't fix it. Changes can be made sonically to the preamp by replacing coupling caps etc.
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.