A good place to start will be with the HD500x in "studio/direct" mode. Plug the 1/4" out from the POD into the FX Return on your AC30.
As silverhead mentioned, in terms of connecting external gear like the Boss looper, add an FX loop block. Double check the FX loop settings / switches, to be sure the levels are set properly.
There should also be some control in the FX loop block, for boost/cut on the send / return.
Ideally, you would want the FX loop with the looper in it LAST in your signal chain. You may need to experiment a bit with this. You might want to set up the rig as mentioned excluding the looper for starters, and dial in a sound you like using a blank patch, and building it from scratch.
Audition some of the amp models until you find something that makes you want to play guitar, and zone out for a bit. Then, doing an A/B comparison between the HD500 fx loop on and off, get the levels in the FX loop block and the external Boss unit, so whether the fx loop is on or off, you get equivalent volume.
In terms of the FX loop block, the "Send" value will let you choose from 0dB, and from there you can cut, to as much as -80db. For the "Return" value, it starts at 0dB, and will let you boost to as much as +24dB.
Here is a .zip of a patch, four versions of same patch, as just a starting point. Plexi, AC30TB, Tweed, and Blackface, amp models. Same FX settings in each. Fx loop set as last in chain. Plus reverb, delay, compressor. The footswitch controls are as follows:
FS1 = Tube Compressor
FS2 = '63 Spring Reverb
FS3 = Digital Delay
FS4 = HD 500 FX Loop