Calypso and Rhea.....

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.

Heat is a big enemy of electronic components. Especially true w/tubes that run hot to begin with. Also low level signal in phono stage very susceptible to alteration by mag fields and RFI from the line stage.

I have a Janus. In the manual it says that there has to be a minimum of 3 inches of space above the unit. My unit gets a nice warm operating temperature even with this much clearance so I think it is not possible/advisable.

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.

Don't do it!