I had the same problem with solid state amps. I stacked them using 3/4" stock aluminum, 1/8" stock plate, that is routed and sandwitched with marbles. This works similar to Symposium Rollerblocks. Not only does it help with heat dissappation it also helps with sound quality. The top amps are smoother with less noise--it might be even more significant with tubes. I also painted the aluminum black for cosmetics. You can see how it looks at:
http://home.mindspring.com/~rrbird/_uimages/amps.JPG
http://home.mindspring.com/~rrbird/_uimages/amps.JPG