[MrPorterhouse]

The fact that its drastically different from your front soundstage isn't such a concern, but what is a concern is the response of your surrounds. While its true that surrounds don't carry a lot of impact, and its also true that you can set your crossover to 100Hz to help, you still can't get away from some of today's agressive surround sound tracks. There are some movies that demand a quality speaker, or at least in order to take full advantage of the surround experience, you'll appreciate a full surround speaker.

I don't know how that driver will extend when put in a box, so if you can get extension down to 65Hz, then you'll be good to go. Even if you select a 100Hz crossover, then frequencies at and below the crossover will still be called upon by that speaker, and the more capable your speaker is, the more natural the rolloff and easier to blend. Considering the capability of your mains, you might want some capable surrounds as well.