Layering 2 delays with 250ms and 500 ms delay time.
Compression both before and after the amp/cab model or even after the reverb.
Resonance and EQ adjustments.
Guitar pickups and string height.
A trick I do for playing live is to place an additional compressor after the amp/cab model and adjust it where single notes sound just as loud as a full chord. This let's me cut through a mix and gives me a bigger sound for leads.
Here's my patch. It may give you some ideas.
I also use Marshall amps, a pair of JCM900s that I spread apart for live stereo effect. I also play hard rock metal.
For me the PODHD was a struggle for several months. The Line6 EQ sucks so bad I went out and bought a used dual band EQ, that helped a bunch. There was more than a few times I thought I had wasted my money and was going to get rid of the PODHD. In the end the light bulb finally turned on and I got it working the way I wanted.
That's the bad part. The good part is, once I got it kind'a figured out I can't go back to other products that don't have the flexibilty of putting anything anywhere in the signal chain. Also the tone is definetley HD but it's hard to tell until you get it sounding the way you want.
If you decide to give it another try (I think you should) then here's a couple things that will hopefully help.
I had posted my tone on another thread to help someone else in a similar situation. Here's the links again. I plug my guitar into the PODHD and PODHD into the Effect Returns of my amps. I don't use the 4 Cable Method.
Download these two tones and see if it helps point you in the right direction.