I would say the amount of heat is direct proportional with the current passing through those tubes. Or at least this is how I understand it. So if you use 6SN7 + 6AS7 you will generate more heat than using for example 4 x EL32 (both drivers and powers). But both configurations should be fine since the amp supports up to 6.5A and therefore should be fine also with that amount of heat coming through them. Regarding the impact on sound signature, I usually think that the output tubes have a bigger impact on lower frequencies and the drivers are somehow taking care of the rest. But I do not really know if this is an assumption or it really has an electrical explanation. Please also be aware that I never tried Elise with stock tubes since I already owned some when buying it. So I have no idea how the stock tubes sound... But I have here a pair of 6H8C which they usually ship as powers, as far as I know, and they are quite OK... even if they are not exactly my cup of tea. But please take my post with a grain of salt since this are pure "speculations" based on how I perceive things.