McFarland (and Horseface), There is no harm in placing a small fan behind a tube amp and draw air accross the entire amp. I think the no-no is putting a fan in front of the tubes themselves and forcing air on the tubes.
Tube amps need no fan assistance unprovided by the manufacturer if you are running it in an open area with sufficient ventilation under, over and on the sides of the amp. If you need to enclose an amp, simply measure the temp in the enclosure close to the top of the amp. So long it is within normal temp ranges for your personal living it shouldn't be a problem for your amp.
Unfortunately, in this situation, Horseface has a SS amp which generates some heat and interferes to some degree with the ventilation under the tube amp (I'm not sure how hot the Levinson gets but its a heavy duty amp and the competitors models I've owned there was a fair amount of heat to the touch. He has 6 inches clearance above the top of the tube amp. Under normal circumstances I would think that would be, marginal but adequate (he could check the temp at amp level and see how much hotter the air was there to be sure. But IMHO, his piling up the two amps has the potential to overheat both amps. I think his solution by placing small fan(s) behind the amps drawing air away from them is an excellent idea. It might be overkill, it might not, but there is no down side if he can control the fan noise.
At least that is my experience with tubes and fan YMMV.
