Why should only those of immodest means get to play around with gear? After all, that's part of the fun. I can think of a number of interesting and enjoyable uses for your cash:
1) as you mentioned, a new dac
2) I beleive you can seperate the pre from the amp in your NAD, correct? if so, maybe an older tubed pre like a conrad johnson or audible illusions
3) you could get a rega planar 3 turntable (don't know if the NAD has a phone stage or not, but you could get one of them cheapy battary powered ones)--talk about upgrading your sources!
4) if you're even somewhat handy with a soldering iron, you could make your own cables or, if you wanted, the bottlehead tube pre
Lots of choices. Have lots of fun.