I'll assume you know how to set up the four cables for the four cable method. There are a couple of hardware switches you need to set the HD500 to.
Firstly the 1/4" out switch should be set to line. The output switch for the HD500's FX loop should also be set to line. And within the POD's settings you need to set the output to either combo power amp or stack power amp. You'll also need to make sure you create a patch with an FX Loop block enabled. When using a HD500 like this you should have its master volume on full.
I set a patch up like that in a new playlist I named 4CM, calling the preset "Empty 4CM". I can then copy it to any other preset position before creating new settings.
Now as things stand this setup won't exactly sound any good. And that's what I've been struggling with. Until a lightbulb went off in my head. The problem is that you want/need a line signal to the power amp, but you want a stomp level signal fed to the pre-amp. With these settings the HD500 is going to be sending a line signal to both - and the result is a less than pleasing lack of dynamics and richness in the pre-amp tone.
This is actually pretty easy to solve. You just need to go into the setting on the HD500's FX Loop block. On the send setting you just reduce the send level by 7-10db. I tried 10db at first and it sounded crap through my Bugera V22. I finally settled on 7db as the setting that made my amp sound like I didn't have the HD500 connected to it (which is what we're trying to achieve before adding FX).
The result is that any effect in your HD500 before the loop is sent to the physical amp pre-amp at stomp level, but anything that appears in the amp's FX Loop runs at line level.
Now I don't necessarily want to use this setup when using the HD500 modelling. This 4CM setup is just for when I want to make full use of my amp, I also added my HT-Dual between the HD and the amp, and it sounds great with stacking drives from the HD500 into the HT-Dual.
Now for when I just want to run with the modelling into the power amp I've found I don't actually have to make any changes to the cables at all. I can leave it all set up.
I just switch to the playlist with all my modelling patches and these automatically cut out my Bugera's pre-amp because they don't have the HD500's FX Loop in use. The output of the HD500 is already running straight into the Bugera's poweramp. There's one thing to be careful of when doing this though. Some amps have their FX return AFTER the master volume, which means all of a sudden the HD500's master volume becomes the master of the amp too. So best to turn it right down before switching to a simple HD500 > power amp patch.