I owned an ESP KH-2, that had EMG-81's in it before I traded the guitar for my JEM, which has Evos, and I still use the same amp (Crate GFX1200h head and cab, and although I've played them both through tube amps, the very basic differences still applied, in my opinion). Both guitars are similar, alder bodies, maple necks, double locking trem.
Now, this is just from my memory but...the 81 -to me- was brighter than the Evo, and seemed a little more thin but not by much, it also crunched more whereas the Evo has a slightly thicker more compressed sound, which obviously isn't conducive to a crunch sound, and an overall sound characteristic that I refer to as a 'brassy'--again, in my opinion.
Obviously, the 81 has more output, and is perhaps a little more quiet hum-wise, though the Evo isn't a low-output pickup by any stretch of the imagination and really isn't very noisy at all for what it is. They both are capable of producing nice pinched harmonics.
Overall, I thought the 81 was a great deal more dry and flat sounding, and perhaps not QUITE as expressive as the Evo and didn't seem to let sound of the actual guitar come through as much.
As with anything involving sound it's entirely subjective, though I tried to be as 'scientific' about the comparison as I possible. I hope this helps.