Although the K701/2 have a low Ohm impedance, they are insensitive and require more power to get them going well.
FiiO E10K is the best of the bunch for driving the K701/2. Neither the Xonar U7 or DGX has a headphone amp that's good enough for those headphones. The SB Z would fare better, but it's not ideal and not as good as the E10K's amp.
The caveat is that the E10K has no surround sound processing for headphones. It's just a stereo DAC/amp, unlike the others which are sound cards and have effect processing.
For a budget set up, there are 2 or 3 options to go for:-
1. Used Xonar STX for around maybe £70
2. FiiO E10K and use Razer's free headphone surround sound software
3. SB Z, which may be OK until you can afford something better
Interesting to note what nonamenonumber said about the E10K not driving the K7XX as loud as he'd like. E10K has a more powerful amp than the SB Z, so going by that, the SB Z would be rather inadequate.
That does make some sense though, because I remember someone on a different site saying the SB Z didn't drive his K612 all that well. He described the sound as being like 'flat cola'. IIRC, the K612 are half as demanding as the K701/2/xx, despite having twice the impedance at 120 Ohm. Impedance doesn't tell the whole story.
Not to say that the E10K isn't a good choice though or won't drive the K701/2/xx that loud. What any one people considers to be loud differs from person to person. A friend of mine listens what seems to me to really loud volume with his headphones. Far too loud for me and I'd end up with either a headache or knackered hearing.