A couple notes...
cowboy has a small/medium sized room, 3000 cubic foot - that'll help ensure a less sensative speaker fills his space.
cowboy listens at -10 - the Jamos have already proven to sound great at that volume, in my recent experience, in a room much larger (say 4,500 cubic foot if I were to hazard a guess of tatersly's room size). Cowboy has a nice quality AVR to start with. cowboy with the jamo/captivator combo could easily run his single subwoofer 6-8dB hot (my typical preference) from the overall AVR volume of -10 and easily still have plenty of subwoofer headroom and clean sound.
When/if cowboy decides it's time to upgrade he pretty much already has the best available ported sub outside of DIY, so if he wants more sub power, add a second captivator and he's still top of his game and can even can use the same single EP4000 amp for two caps.
As you point out ---
cowboy will feel the jamo's are his limiting factor on SPL long before the sub as he starts to turn the volume up to reference level playback, but at the $400 price point. He can use them for a year and resell them on craigslist for a minor loss, or move them to a secondary house system, when/if he decides he wants more. They are a surprisingly good sounding set. During my audition, I kept thinking how in the world do they make and ship and markup and sell a set of five speakers that sound this good for $400. It still baffles me. If I had to guess I think Vanns is Jamo's attempt to enter into the US market. When I bought my two Jamo D7 subs and had the amp issues I was trying to find an authorized repair center. There is only one in the entirety of the US. It is vanns. Jamo is big overseas, but virtually unknown here in the states. I think they are trying to break in and so offering these speakers at what effectively must be essentially just their cost.
If I hadn't heard them I would cringe a bit at the cheap speaker recommendation, but I don't think you can think of these as $400 normal price speakers. I'd say consider them more in line with a typical $1000 5.0 set. I'd wager they sound pretty similar to my Wharfedales, and also to the Crystal Acoustic set I recently reviewed, and my brother's $2000 Klipsch 5.2 system he bought off newegg sounds no better. Again they aren't world beaters, but they are especially nice for the money. The bonus in this setup is if he buys midline subs and midline speakers, then he has all midline stuff, and if he should decide to upgrade he'd have to replace everything. In this setup he buys a top shelf subwoofer, virtually nothing better...(competing products tie or win by very small margins at this level among when comparing items like the Orbit Shifter, Submersive, Cap sealed, etc) so there won't be a desire to swap out subs and take a fairly significant hit in a year or two. In fact the caps have only been sold in about two instances that I know about, carp and buckamdman, and in both cases they have fetched close to new value. And if he decides to swap out the speakers at some point he does so at a very minor loss because of the cheap price of entry on the set. It does seem a strange recommendation, and a bit unbalanced, based on price, but it would be a really fun setup for movie watching. If he saves up for a new set of speakers, while still wanting something budget minded for the mains, I would recommend the ED Cinema 12 speakers --- For the price they seem a real gem. I'd recommend a front soundstage built with those but then he loses all his sub money.... So a new set of mains could be next years planned upgrade if he finds the jamos aren't up to his spl desire.
Cowboys, you got a lot of good advisors in this thread. I hope more continue to post. I certaintly don't have the end all advice. I'm just one voice. Weigh it all and choose your own setup.
Do you live close to anyone in the subwoofer demo thread? If so audition some equipment and see what you like.
There is a companion thread created for speaker auditions by brandonnash linked as well in the first post.