Buy a mixer (if you haven't one already) and raise the input level of the guitar or (as a last resort) add a Tube Comp before the amp in the patch.

Start with your pickup height.  This is where the amount of gain starts and if you are not getting enough here you are simply trying to compensate through the rest of the chain.  You want it as close as possible without getting any artifacts or weird frequency response.  This also differs depending on the pickups you use, so don't just measure someone elses and try to copy.  Use your ears.

Also, make sure you have the inputs 1 and 2 of the POD on Guitar and Same respectively.  If you have the second input set to something else you may lose gain going into the amp depending on how you have the FX chain set up.

The Park and the Plexi, although they sound great, do not have enough gain alone to create the sound you are going for.  The closest I have found is the Soldano Crunch.  If you use a tube screamer in front of the Park or Plexi, as described above, you will get the sound you are looking for.  You have to remember that EVH, for example, had a somewhat special Marshall Plexi that he used and everything was cranked on 10 (dimed as he said).  For that amp it worked great.

Probably what you are going for is the sustain you get from having a certain amount of distortion, not the distortion itself.  That's why I would love for Line 6 to add a Friedman-type amp to the line up.  Basically this would be a Plexi with an extra gain stage and a little better EQ.  Perfect for what you are going for and what I have been searching for.