I had fans mounted inside of a cabinet to cool a large SS amp. You can find very quiet small computer fans in a variety of sizes that can be powered by DC from a wall wart converter. If you use a lower DC voltage, the fan speed can be reduced resulting in quieter operation. I used a variable voltage converter from Radio Shack. Ultimately, I moved the amps outside of the cabinet because I didn't care for the dust the fans caused to accumulate. If there is any way you can support your tube amps on amp stands outside of your rack, I believe it will be better for you in the long run.
cooling mono amps
I have two BATVK-55 mono block amps sitting in an Audiov Zirconia rack. They give off a lot of heat when the ac is not on. I read threads about pulling the air away from the tubes and have seen 5.25" fans on line, two configured in a 19" panel. I was wondering if one of those mounted at the back of the rack, one behind each mono block, will suffice? Should I include a thermostat? Is ther another quiet fan I should be looking at? I would appreciat your comments and suggestions. Thank you.