voted subjective - good topic btw
i think there is very little correlation between complexity of music and heaviness of a music piece. as an example, joe pass had a complex chord-melody style, but i would never call his music heavy, especially in comparison to artists that use heavy thick distortion + blastbeats. alot of artists use insane modulation, key and time changes, contrasting dissonance and consonance, upper, altered chords etc, but i would say the standard breakdown would be much heavier than that. i know opeth have alot of change and deviance but they are far from the heaviest band and have never been one of the heavier bands.
i say that there are certain tones that sound evil to the human ear, along with dissonant tones that have a natural yearning for resolve. volume, pitch, timbre, speed etc...all these come into play when considering the heaviness of something. none of these work very well on their own however, and there are always exceptions. you could play john coltrane as loud as u want and it would never sound heavy, maybe because of the unique timbre of the sax, or pre concieved views on whats heavy and whats not. i think that when you drop the tuning of a guitar down it gets heavier, but if you cross a certain line into bass territory it loses the heaviness and the evil and becomes groovey. timbre speaks for itself, i dont know anyone that thinks an archtop guitar on squeaky clean sounds brutal (although some jazz guys will probably try to argue this, as ive seen before - they are elitist fucks).
complexity is good for conveying other emotions but i dont think heaviness is one. im also adamant that fifths are not the best way to convey heaviness in music. complexity has its benefits elsewhere but i think that the general quality of sound and its intervals etc have a distinct heaviness
nice topic, hopefully it will evoke people's opinions. very hard to have an outright answer