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IMHO without loud, driving percussion (a drum set), loud, distorted (screaming) electric guitar and loud, distorted (screaming) vocals, it can't qualify as heavy metal.
If you disagree that loud distortions in vocals and electric guitar are always musically disgusting, then we really can't have a conversation here. I've spent far too long working far too hard in the appreciation of truly beautiful music to throw that all out in favor of some popular urge for the diametrically opposite qualities.
As for loud, driving percussion (drum set), I would perhaps allow some figment of possible redeeming value, but I'm not quite sure of the appropriate application. E.g., whenever I've seen a drum set installed on a stage, even with a symphony orchestra in front of it, I have been not too shocked to find the reason for its inclusion has utterly nothing to do with symphonic orchestral sounds.
There are various folk genres where the presence of prominent percussion is an integral part of the artistry and tradition of the whole. Therefore, percussion, per se, cannot always be found "inherently sinful." However, for some persons, the hearing of and being subject to incessant drum beats can only excite their base passions, and before they know what's happening, they might lose control of their own physical urges. To them, therefore, willingly being in that environment is to put themselves into the near occasion of sin.
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