tube heat

Open air ventilation is all you need, no fans necessary.

Don't put them in a cabinet with doors. If you must, remove the back of the unit, spread the shelves as far from each other as possible and keep it a few inches off the back wall. But really, don't do it. Cheers,
Spencer

Get a hold of some O2 Cool battery powered fans and aim them at the output tubes. Problem solved.

Tube amp should be on a stand for heat dispersion.

Geofkait-let's agree to disagree...Battery powered fans can work but I would point the fans away from the amp to passively pull air over the amp.  I've heard noise from the air blowing.  Also, by blowing on the amps you will get dust build up over time.