Hey guys.

I've bought a Line 6 POD HD500X recently, and I've had 2 gigs with it so far. But both times, when I asked the soundman if I should plug the POD directly into PA, he said 'no, it'll sound bad, just plug it into the amp'. Both me and the second guitarist use my POD (separated inputs and outputs) and I use only distortion and effects on my POD and turn off the cabinet simulators when plugging into the amp, so our sound tightly depends of the type and quality of the amps in the venue.

Is this how HD500X should be used live (it still sounds very good), or should I insist on plugging into the PA next time?
tell him to shove it in the ****ing pa, thats what its meant to do for live sound, obviously more common on amps, but you should have NO problem getting it into the PA.
Yeah, tell the sound tech that you want to use the amp sims on the POD and that you want to go straight to the PA. There's no reason not to do it.

The best live sound I ever had was at a bar gig where the stage was pretty cramped so we couldn't bring our amps and the bar owner supplied an electronic drumkit (since our drummer's regular drumkit takes up a lot of stage space).
Guitars and bass went through multi-fx units with amp sims to the PA, vocal mics and drumkit straight to the PA.
The sound mix was incredibly well balanced, clear and tight.
Echoing the others here, but go direct front of house, don't even bother with an amp and cab. Just make sure whatever presets you are using sound good for the stage. Try mixing them yourself at the highest volume you can beforehand and you'll have a pretty good idea of how it will sound on stage.
It's your sound guy who has the issue.

We found, when we were first playing with modelers, that it took some time working with the sound guys to get them onboard. Once they had it dialed in, they were actually pretty excited about the whole thing. When they realized that there was so much less instrument contamination in the vocal mikes, they embraced it whole heartedly. I think they secretly like the extra control it gives them, too.

You might get together with the sound guys at your gigs (upcoming and past) and spend some time with them and the Pod outside of normal hours. Bring pizzas and beer.
What's the difference between the mix level of a real amp and a modeled amps? Shouldn't be too hard.
The sound guy is a Muppet. The whole point of modelling and multi-effects pedals is to bung them through the PA. Think about it. Playing through an amp will adjust the sound that's supposed to come from the thing.