If their monitors and stage volume can bury your vocals (or whatever else) through a PAS, then you guys are probably playing so loud that it wouldn't matter what you were using.
If the band is really loud, you will likely have to run the analog line out from your PAS to the main system, tell the sound man not to put you in the monitor mix, and add a little reinforcement (over the top of what the PAS will be projecting to the audience) to the FOH.
Chuck, I agree with Clark, but remember that a major benefit will be that the other folks in the band will also hear you really well (but you won't hear them nearly as well). You may not even have to be in the mains, the system gets loud.
I've played three gigs in the last month where I used my PAS and everyone else used a traditional setup. In this environment, I do not put a signal into the PA, I run my vocal mic and bass into the PAS and trust it to do the job. If I can hear myself well, I know it's getting out there.
The others hear you well, but still have the same old problems, if they start to crank up, I just don't turn up. I hear the others slightly better than I would if I wasn't using the PAS, because my sound is out of the confusion (monitors mostly), which makes it a little easier to focus in on the others, especially if instruments are not hot in the monitor mix. The contrast is a bit annoying, because I have a full sound and the others sound two dimentional and lack focus and distinction in comparision. It sounds like you're not blending, but out front, with a reasonable PA it sounds OK, but you can tell the difference, since the PAS instrument and voice is every where in the room.
If the sound person or the guy who owns the PA wants to run you through it, have them listen first with just the PAS. This usually will pacify them.
I'm gonna sit in with a friend's band this weekend. I talked them into letting me bring my PAS so we could try it out as a monitor system. They're a 3-piece rock band and they all sing. I can't be sure until I try it, but I suspect that the PAS has a real chance of replacing their PA system! They'll be running a kick drum, electric guitar, keys & 3 vocals through the reuglar PA. My guess is that the Kick may have to stay in that PA, but nothing else. Anyone here try this yet? Results? Thanks in advance... I'll post back and share my findings. Maury