A lot has to do with the voicing of the amp, and how much of your guitar's tone it actually lets through. Also, to get that real thick metal PH, it can sometimes be easier if you've got some faily thick strings that are tuned down kinda low, so they're a little loose. A whole step down with the low D tuned to C can usually produce the PH's you're looking for. Also, fret the Low 'E' string (or whatever you've got it tuned to) between the 1st and 5th fret. That's usually where you'll find the "heaviest" sounding PH's. Also, be sure to use some nice and wide vibrato.