the X1's are a great headphone, and depending on your listening device you may not even need a DAC, but it depends on your situation. Generally speaking, the DAC won't make that much of a difference, unless your source has a REALLY crappy one. The headphones will make the biggest impact, so its best to start there and then if you think you need it, you can buy a better DAC.
The stock cable on the X1 is known for having a high impedance, BUT again, YMMV in regards to how big of a difference it makes. If you read through the primary X1 thread, there's some debate about it. Some say it makes a HUGE difference, and some say otherwise. If you have some extra money, you can try getting a different cable and seeing if it makes a difference for you. The Fisual is a great choice if you're in the UK, for those of us in the US the standard is the V-Moda replacement cable. I have the X1's and enjoy them immensely, both with the stock cable & with the v-moda.
Also just wanted to mention, if you do go with the X1's, its pretty much unanimous that they improve quite a bit with burn-in, so definitely give them some.