Not sure if you've got a FH1500 yet, but mine arrived this week and so far I'm really impressed. I also have a HD Pro X which I've been looking for suitable amplification for. So far I've relied on keyboard amps which are decidedly sub-optimal (in my experience).
I've been a little jealous of the Helix users who have a dry output option to give Wet/Dry/Wet sounds. However, you could do this with the HD500x with a little effort.
I run the guitar into the POD and then the effected(?) signals into the XLR inputs to give the 2 wet components. Then run the 'dry' out of the POD (which is essentially the same signal as from your guitar as it doesn't hit the signal chain) into the guitar in on the FH1500. I set up an 'un-effected' version of the amp to get the 'dry' component on the FH1500 and then balance the sounds up to taste. It works really well as you have the same amps (more or less) on the FH1500 as you do on the POD. Not sure what connections you have on the 500, but if you split your signal into the 500 and run one end to the FH1500 - you should get the same result.
Once that worked, I set up MIDI signals from the HD Pro x to flip to the corresponding dry amp models on the FH1500. In its default mode, the selected patch on the POD HD will trigger select the corresponding patch on the FH1500; i,e, selecting 1A on the pod will select 1A on the FH1500, but you can override that.
Works a treat. We have an outdoor service coming up - looking forward to it.