My only suggestion would be to definitely try before you buy. There are a lot of flabby sounding / slow responding 18's out there and you probably don't want to get stuck with one. I've heard great things about the SWR Big Ben and it's fast response and very tight bottom. Can't say I've ever used the Carvin version.
Question: from what you said it sounds as if you're planning on running 3 cabs off of the same amp. Can you amp handle that? If each channel was 4 ohms (assuming that the Hartkes are 8 ohm if I remember correctly) then you should be fine and you'll probably want to get a 4 ohm 18 to use up all of the available power on that second channel.
I'd wait to see what other people here have in the way of comments, but personally, I wouldn't buy an 18 (or any cabinent for that matter) without hearing it first. That's my opinion though. Also, I don't know how many watts your QSC is, but remember that most 18's (that I've seen anyway) aren't very efficient and guzzle up your available power so you'll want to have plenty available.
Oh, thought of one more thing, though this really just has to do with how you want to sound. From my personal experience with a home theater subwoofer (not the same, I know, but it does apply), I'd say that if you set the crossover at 100 hz, you'll not end up sending much over to the 18 at all. Sure, you'll be getting the root of most of your notes, but I'd personally set it slightly higher to around 200 or so hz (that's what I did on my home theatre sub when listening to music and it brings out the bass part tremendously). At any rate, do whatever works for you.
[Edited by White_Knight on 08-21-2000 at 08:30 PM]