When DEF LEPPARD emerged a few years ago, they were hailed as the freshest young headbashers on the heavy metal scene since AC/DC. Meaning they're capable of more than just standing around screaming and sweating. With their third album, "Pyromania," the promise is still there, but the screech exceeds the grasp. Meaning they sound a lot like AC/DC, only with more practice. Part of the problem is the same one their colleagues KROKUS have on their newest release, "Headhunter," which is that they seem unable to assert their individuality, either vocally or riffwise, for more than a few bars per track. Such numbers as "Comin' Under Fire" (from "Pyromania") and "Ready to Burn" ("Headhunter") bear such unmarked resemblance to the AC/DC catalogue that the two groups appear to be conceding subordination rather than competing. Still, both have an interesting way of dealing with the problem. Def Leppard's consists of tagging on lush-sounding intros, as on "Rock! Rock! (Till You Drop)." It's received sound, computer programming that embellishes the songs on vinyl yet can easily be disposed of in a live context. Krokus, on the other hand, has thrown some humor into the mix. That's what makes such songs as "Eat the Rich" and the title track such a scream, so to speak. They also display a rudimentary knowledge of dynamics, particularly on "Screaming in the Night." Which, when you think about it, is what distinguishes the heavy metal men from the boys. Then again, if you're thinking, you've already blown the premise. ON RECORD, ON STAGE THE ALBUMS DEF LEPPARD -- Pyromania (Mercury 810 308-1 M-1). KROKUS -- Headhunter (Arista AL 9623). THE SHOW DEF LEPPARD, KROKUS -- Friday at 8 at the Capital Centre.