I would like to point out that an understanding of musical theory is not necessary for playing technically complex music. Lots of people do very complex things yet could not tell you the slightest things about the concepts underlying what they're doing.
That being said, technically complex music is not necessarily good music, either. Muddy Waters played some of the most simple, direct music and yet people like Ian Anderson of Jethro Tull had a huge hard-on for his music. Conversely, you could find all sorts of avante garde music which, while technically complex in the extreme, is about as pleasing for most people as running one's fingernails down a chalkboard...
Also, when one speaks in blanket terms about "heavy metal" it's really hard to accurately describe such a huge category. After all, this is a term that includes everything from '70s, down-tuned, power-chord-driven fare like Black Sabbath, to the '80s LA-based glam bands like Motley Crue, or the classically trained extremely intricate and sophisticated work of Ingwie Malmstein, and a whole lot more.
Finally, I really have to say that the idea that objective standards can be applied to a subjective subject is ludicrous. Music is largely cultural, otherwise Eastern music wouldn't get on Western listener's ears w/its scales consisting of 1/4 tones when those ears have been acculturated to 1/2 tones.
OTB