Well, I didn't try the XPT amp-less, but I believe its impedance is 64 Ohms and its sensitivity is around 100 dB, so you just might be OK without investing in an amp. Out of your computer and perhaps an Ipod you are probably fine. I'm a firm believer in trying to get the very best sound possible without expending a lot of money. If I were in your situation, I would buy one of the headphones in which you're interested. Listen to it for a while and then if you decide on a DAC/amp, keep your eye on the classifieds here. For example, the Audioquest Dragonfly has really dropped in price, and there are always the E-07 and E-17 offerings from Fiio.
Well, I've listened to quite a few low-fi and mid-fi HP's, and my feeling is that the Chinese XPT is pretty competitive with the likes of the ATH M50, etc. Listening is a very subjective exercise and no two people will hear exactly the same nuances of bass, mids, highs, and sound stage. The brief time I had with the XPT left me impressed, but that was in conjunction with a pretty decent DAC/amp. Good luck and welcome to Head-Fi. Your ears will be the final judge.