I am a culprit (my phono stage is resting on top of my preamp even as I type), but the best advice is unquestionably don't stack any audio gear. With tubes I suspect this is doubly true.
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Hi RWD! I don't think it is a good idea. Not only for the reason of heat dissipation but also because of the way the tops of these units are attached. While plenty solid for their intended purpose, the tops are not rigidly fastened. It would be a pain if you want to change tubes.
You could easily fashion a shelf that would be wide enough so that it was supported on either side of the lower component but still rise enough to leave clearance.