Floor Standing Speakers That Would Sound Good With the NAD Integrated Amplifier.

I completely agree with akg_ca in that with certain gear, particularly in the mid-hifi range, you'll reach a point of diminishing return when choosing speakers. I've experienced that with a couple of integrated amplifiers that I've owned in the < $1500 range and experimented with my own and friends' speakers, some of which were an order of magnitude more expensive than my integrated. They just sounded different and in some case better but not necessarily jaw dropping better. I must admit there was one exception and that was when I hooked it up to a pair of Charney Alerion single driver horn loaded speakers a friend of mine used to own and it simply blew me away. So I guess extra efficient speakers might not follow this logic. If it were me, I would reconsider my budget and audition as many speakers as possible before making a decision.

What's your ultimate system goal? Are you married to your NAD?

Personally, it's aways worked better for me to choose speakers I love in my price range and that work in my room, THEN find the amp that best mates with the speakers. (Yes, I think you should fall in love with your speakers - otherwise, why bring them home?) I am the patient sort, willing to wait to upgrade to get optimal sound. I've never had the budget for an "I want it all and I want it now" system. 

And that's the thing - it's a *system*, and synergy is important.  The speakers have always been my key investment and were allotted the largest slice of my budget, followed by front end (be it digital and/or analog), then amp, then cabling  etc (and that's a whole 'nother subject). 

I'm totally onboard with integrated amps, There are some truly outstanding ones available in various price ranges - it's like a golden age of integrateds, be they they SS, tubed or hybrid. Once upon a time, I had relatively expensive speakers and separates and loved them. But I am very happy now with an integrated amp and plan to stay that route.

Your NAD may not be not the last word in transparency or upper frequency refinement but it's quite enjoyable at its price point and capable of driving a wide variety of speakers. Consider auditioning reasonably efficient speakers in the $1500-$2500 range, enjoy them with your NAD for awhile, and - if you feel the need - put the money saved toward trading up your amp based on your new speakers. Also consider buying used/demo amp for bang for buck.

You don't say what you're looking for in a speaker or your music preferences other than full range and compatible with your NAD. But I concur with the above suggestion for Vandersteen 2s and would add the GoldenEar Triton line to that list (caveat: we have the Triton 3+, $2500.) I'm a big fan of full range or as close as you can get.  You hopefully have local dealers where you can audition both full range and stand mounts - you may even be able to bring your NAD to try with them. Don't be in a hurry and have fun. But get out there and listen.

My personal experience is that I have an Nad C-388 integrated amp and I'm using a pair of Qacoustics Concept 40 floorstanding speakers and the combination is great. Great detail, tight bass and they are only $999 for the pair. Qacoustics put out some great sounding speakers for a very reasonable price. The build quality and gelcore technology they use makes for a distortion and vibration free speaker. You can't beat British sound, imo

I had a NAD  C356BEE and C272 power amp that was bi-amping a pair of PSB T6 towers- new model equivalent would be PSB X2T.  
Deep and impactful bass- the bass response on those PSB speakers is incredible and your amp will deliver the goods.  Midrange was excellent- a little forward but excellent with vocals and jazz.  The upper frequencies were again- quite excellent.  PSB tunes their titanium tweeters to sound incredibly detailed and yet natural.  No subwoofer required.  

The latest Monitor Audio Silver 7 300 or 500 are spectacular values and a good match for both your amp and room size. The dual 8" 500s go a bit lower and louder than the otherwise identical dual 6" 300s, but the 300s work better closer to rear walls. Both have dual tunable ports that are a big help in tuning below 200 hz when rear walls are a consideration. In your room, I might favor the 500s on the 17' long wall, the 300s on the 12' wall. Either way, you have to double the spend to even get a chance at better sound from most other brands. I bought the 300s because of space and to get a bigger soundstage than my LS-50s and am extremely pleased