I've been using my HD500 live for about two years now and I've been tweaking (and tweaking... and tweaking) my high gain patch since day one. I've read tons of online discussions (many of them right here in this forum) and guides about how to get good high gain tone out of your POD HD... like this one for example:
This guide is extremely informative, but since I'm not a sound engineer it's, frankly, a bit over my head. I've got a much simpler guide to getting an organic high gain sound (mine is dialed in to sound kind of hair-metal-ish) that I recently put up on my blog:
Theres also a link to download the latest incarnation of the patch that I use live, straight into the PA. Multiple sound guys have told me that I've got the best direct sound they've ever heard, so I think I'm doing something right at this point.
I hope this helps some folks out there in POD-land who've been having trouble with dialing in hard rock and metal tones.