I too, am in a small room of almost the exact same dimensions as yours (11X12x8). I've tried a number of speakers with varying results.
I should emphasize what Lthkeepr mentioned regarding bass ouput. Full-range speakers may be tempting and may sound terrific at the dealer's showroom but could result in boomy, annoying, one-note bass in your room.
You didn't mention whether you're looking to only buy new or if you're open to used as well. For new, I would STRONGLY recommend the Magnepan MMGs. $550 shipped to your door directly from Magnepan. Your NAD will have no problem driving them (unless you like to listen at headbanging volumes - in such case the MMG is not a good choice anyway). For the money they cannot be touched.
There are a fair number of monitors that would work well also (Like the Epos M12 that were mentioned), but you need to be prepared to spend another $200 for some quality stands. Monitors are great, but the issue of stands brings your budget down to $500 ($450 if you're having them shipped) and there's not much to choose from at that price point.
The Thiel 1.5 is a nice speaker too, but I wouldn't recommend it with your equipment. They can be very revealing of upstream components and are probably not a good match for your NAD.
Based on your post, I get the impression that you're not one to buy/sell/trade equipment regularly and want to find something that you can be happy with for a long time? If that's the case then I really can't recommend highly enough that you give Magnepan a call. For your $550 you're getting a brand new speaker with a warranty that includes shipping from a VERY reputable manufacturer that's not gonna' close its' doors tomorrow. What's more is that there's a 60 day in-home trial period, so if you don't like them (I highly doubt it), you can simply return them for a full refund! Or, if you love them and want to move up the Magnepan food chain, you get full trade-in credit during the first year. You really can't lose.
I've been in your situation. Several years ago, I was looking to replace a pair of Boston Acoustics T-1020s. They were not the last word in refinement, but when I set out looking to replace them with a budget of about $1,000, I was shocked and discouraged. I didn't hear anything that was worth getting excited about. Some of the older BA stuff is really quite good. I finally decided that the only way to make my $$ stretch was to buy used. If you're willing to take the risk, it's hard to beat some of the deals you'll find on this site. If you want new...call Magnepan and order your MMGs today!
Best wishes to you,
Dan