Definitive Technology In wall theater is amazing and they are one of the very best dollar for dollar in wall systems around. Very clear and dynamic.
Forget about the spec's , we all know they don't go that low , your not going to try and send that low of information to them as they will be set to small and redirecting all low end to the subs , yes use 2 in wall subs please. They work much better in pairs especially for the room you listed.
I like the first system much better. But I would take it a step further and use all 7 channels with UIW RLSII's all the way around. It totally rocks and sounds amazingly balanced. I actually just put together a full dedicated theater with the same exact setup but I power it with a Integra DHC80.3 preamp and a Integra DTA70.1 I also use a 133 inch 2:35:1 microperf Screen so the front 3 all live behind it. The UIW's play perfectly through the screen even before calibration.
does the 818 receiver have a true dual sub out's? If Not I might be a good idea to upgrade to a Onkyo model that does or go Integra which have true dual sub outs in the 80.3. Also buy 2 sub amps so you can take full advantage of this setup.