Although I've never been bothered by the heat of my tube amps in the summer - the AC's usually on anyway here in DC*, where it's almost always hot and humid during July and August - I would be wary of keeping my amps in a closet; I'd much rather that they make me a little hot, than for me to allow them to run in an environment even hotter still, which will rapidly degrade their operating lifetime. If the amp(s) can raise the temp of a whole listening room a few degrees, think how much higher they'll raise it in a small confined closet. Ventilate, ventilate...
*[Good grief, I can't believe it took me two long years to arrive at that inadvertant bad pun of abbreviations for air conditioning, The District of Columbia, and alternating- and direct-current!}
*[Good grief, I can't believe it took me two long years to arrive at that inadvertant bad pun of abbreviations for air conditioning, The District of Columbia, and alternating- and direct-current!}