I bought some little fans off of Amazon that are built to cool the power supply of computers. About $30 bucks and work great. I just attached them to the cages of my Rogue mono blocks.
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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.
Have a pair of Monarchy SM-70 Pro as monoblocks. To reduce warm-up time I leave em on 24/7 unless I'm out of town. They're very high-biased class A, so w/o cooling they'd otherwise likely cook themselves into an early grave used this way. I tried a pair of "cooler guys" 120mm fans (listed at BuyExtras.com as "Ultra Quiet 120x120x25 USB Fan with Fan Grill + Rubber Feet" - I just discarded the grill). $14 each. Here was the most important part for me: <10db of noise - a good and hard-to-find rating. Had tried an earlier fan with a 19db rating and it was objectionably loud even in a (backless) cabinet. These were even quiet enough to use one of them on each amp out in the open on the floor. Lesson learned: far better noisewise to use more than one quiet fan than one fan that's twice the noise rating. Another hint though: you do often have to take a fan's given noise rating with a grain of salt. These particular ones I found seem to be the real deal. My goal was to cool the amps to the point that, even when used in a cabinet, their temps would be no warmer than (and if anything slightly cooler than) using them out in the open. Result: the amps, without signal applied, ran roughly 75% cooler in my cabinet than in the open without a fan - much cooler even than my back-up class A/B amp and not far from room temp(!). To keep things from becoming TOO cool and inhibiting proper warm up it was necessary for me to raise each fan up off of the ventilated top plates at least an inch or so to allow some air in under the sides of the fans to slow down their cubic-feet-per-minute rate to dial back the appropriate temp range for me. But, yeah, as long as there's a direct way for air to get in and the same for it to exit and the fan can be placed close enough to one or the other, I would think these would be a good choice. In my rig I've since been very pleased with the results. Hope this helps.
A possible CAVEAT regarding fans: Some years ago a letter to 'Audio Amateur' magazine warned that a C-J MV-45 owned by the writer had the strange habit of sounding very noticeably worse with the tube cover [solidly] in place, compared to simply removing the cage. It turned out that problem was not a mechanical or electromagnetic problem with the cage itself -- it was the quiet fan unit he'd installed causing an overall degradation in the sound (not just some identifiable noise).
Some years later I read something about these fans not always being what they seem due to the use of hidden current pulsing circuits aboard -- the implication being that your DC fan, for example, may not be so "DC" as it appears to be, or as represented in its promotional material. I don't know any more about this, but I'd be very interested to hear the real technical scoop on these devices. How closely did you guys compare? Just asking. If there was an assumption up front that the electrical operation of the fan is benign, could this belief itself have enough of a masking effect -- maybe just in the moment -- so that some small negative could have gotten overlooked. I'm not suggesting the thing wrecked your sound, of course... we all simply accommodate.