only one way to find out...
the peak at 17 hz isn't the problem except if/when you test with a 17 hz sinewave. Its not noticeable with normal program material . For me anyway, the null in the 30's was (and is) more of an issue. I still haven't got multisubs set up. BTW I'm using two dual opposed epic 10s as stands for my mains in a very open room. they are enough for me and more than enough for my wife, if you know what I mean:)
If you sim the CSS sealed, you'll see you need only 12db or so of linkwitz transform to equalize it flat to 20 hz. You will also get boundary gain with your basement walls and floor so with 4 drivers you likely have enough SPL. Simulate vented and you will see you start getting into pretty large box sizes to tune to 20 hz with a reasonable vent.