Do I need a DAC?

The odds are that it would. Whether it is significant enough or not will be what your ears decide.

I agree with Onemug. I wouldn't expect huge differences, but your amp and speakers should be revealing enough for you to hear a difference.

Your amp has a great feature that you may want to make use of. The amp and preamp sections of your NAD are connected by jumped rca connectors on the back. That means you can use it just as an amp or just as a preamp. If you were to get a DAC with a volume control, you can bypass the preamp section in the NAD. Doing it that way would probably make a much bigger difference. If you don't mind a little experimenting, you can try plugging your Airport Express directly into the amp section and use the volume control on your computer. You may be very surprised with the results.

You need DAC. MDHT labs tube DAC is a verryy good DAC only cost about $300-$500 on ebay.

If you're not obsessed with a stereo that sounds good, sell your gear and buy a bose wave radio ;-)

It sounds like to me though, you don't care too much about your sound, simply because you said so, so yes a dac would make your rig sound better but is it worth the $ spent? Only you can decide. Try borrowing a dac or buy one that you can return if you don't like it, otherwise be prepared to spend $300-$600 on something that isn't going to "wow" you over.

"If you're not obsessed with a stereo that sounds good, sell your gear and buy a bose wave radio ;-)"

He didn't say he was obsessed with bad sound.

Actually, I think I can see why the OP was worded the way it was. I've read threads on this site were people would argue like its life and death over things like what kind of rca connector is best on a pair of IC's.