Tim at BKP compensates the tonemellowing influence of the cover in the windings, but usually covers will take a (tiny) bit of treble. BTW, the Aftermath is called to be supertight, so for thrash/deathmetal a great pu. You could also look at the Ceramic Warpig. If you want prof. advice you can mail the BKP-team. They will answer all your questions.