My current primary DAC cost over $1,000. It uses a relatively rare, and quite expensive, DAC chip that has been used in lots of the best and most time-tested DACs. It is a DAC that has been well-received by reviewers and I quite like it.
I have several other DACs that cost around $500 when new.
That said, I am unconvinced I could tell the difference between any of those DACs and any of the following:
1. The built-in DAC in my Pure i-20 ipod dock ($~99 new)
2. My JDS Labs ODAC ($149, new)
3. The much maligned (I actually quite liked mine) NuForce uDAC-2 ($125, new)
4. The DAC section of my Leckerton Audio UHA-6S Mk II
5. Just about any other DAC that has a decent output signal strength
I'm serious. In a double-blind test, I am convinced that just about all DACs that cost more than, say, $100, will be indistinguishable. The only thing that is a bigger waste of money than expensive DACs is expensive cables.
My advice: save your money and put it toward either a Speedball for your Crack, or better headphones.
As for my advice on headphones, I have found the best matches to be Sennheiser HD800s, HD600s and ... a pair of vintage AKG K240 monitors (different from the current K240s). The Senns obviously aren't closed-back, and the AKG might as well be open when it comes to sound isolation.
You could go for a pair of high impedance DT770s (although I have to admit that I'm not a big fan of the stock DT770s), or I also have a pair of German Maestro GMP 8.300D (300 Ohm, closed back headphones) that I'm not using. Send me a private message if you're interested. I've looked around for high-impedance, closed-back headphones, and there aren't a lot of options....
Best,
Adam