The JJ2A3-40 is a nice, robust sounding tube, but, for cost is no object, I like EML 2A3s, particularly the meshplate tubes--very large soundstage and extended top end.
I didn't notice any particular heat issues with the JJ's, but, I was not paying much attention either. Because you mentioned a cover, you should look into whether the JJ's will fit your amp. The tube is taller than a typical 2a3 (the EML's are tall as well).
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