Looking for the good sounding Pre Amplifier

In that price range, I would definitely audition the Ayre C-5xeMP and if you want to start in tube warmth, then the Conrad Johnson Classic 2 SE, make sure you try the SE version, a few hundred more, but well worth the money.  After you have heard these, then you can dabble down the road with a ARC or Asthetix gear if you desire.  It all depends on how your system comes together.

I own a Classic 2SE,  it is a great sounding pre.....

Check out Don Sach's tube preamp (google his name).  It is the best I have ever had in my system.  There is magic in his octal-tube design for under $2K. I prefer a tube preamp/solid state amp pairing for the best of both worlds!  

3k would also get you into some Cary preamps used or new take your pick. The right preamp with the NAD could be a big positive step in the right direction although I can admit there is some NAD I could live with.  

Many nice preamps mentioned above but not sure you're going really know how good they are. Sure they're going to make your NAD sound the best it probably can but those pre's aren't going to be their best on your current amp. No disrespect for NAD; owned some and it's great value but it has its limitations. Have you thought of looking for an integrated? For $3K you can find a great used Integrated I bet can outperform a preamp you add to your amp; plus add what you get out of your amp and add it to your budget.

If you're married to the NAD I'd recommend stepping my budget around $1500 or so. Ton of great preamps in that range that will make your NAD GO. The original MW SWL 9.0SE, SF SFL 1 or Line 1, CJ PV12, ARC LS15 and etc. Tons of great preamps out there. I agree in this range on your amp I would look at tube based pre's and avoid solid state unless you're spending much more then a Pass, Ayre and others are fine. Know all the tube pre's I listed and all are very good. The ModWright by far beats them on the PRAT; so if you want a pre that has drive that's the one. The Conrad Johnson sounds wonderful and has the bigger tub sound but will not have the speed of say the MW or the others. Sonic Frontiers SFL1 is good but the Line 1 is cleaner and more extended with remote. ARC LS15 is a fine pre and probably more like the Sonic Frontiers; more neutral and maybe some will say sound like a better solid state preamp which nothing wrong with that but might make your NAD sound a little bright maybe. I'd look at a MidWright or Conrad Johnson to add a little bloom to your amp. I just wouldn't spend $3K as I really don't think you're going to hear it unless you step up your amp and not knowing the rest of your system you might be spending more to upgrade elsewhere as well.

Good Luck