I only listened to the HFI Range of Ultrasones mainly due to budget, the pro750 is DOUBLE the cost of the DT770 in the UK, so I would expect a vaste improvement in sound quality for the price difference. That being said even the HFI-780 is more expensive than the Beyers and when I was investigating the purchase for myself only sound quality was really in the equasion. At the time I was also looking at the shure SRH-840 which also was on a par both in terms of sound quality and price.
I know location in the world can affect prices greatly, so I would do plenty of research before buying anything. I would make a list of pro's and cons on a list of maybe 5 cans, based on forums feedback where it is more likely to be independent. Look at feedback from more mainstream/bulk supplier such as amazon to find any potential build quality problems (I think with your current selection you will be ok). Then once you have made a choice shop around, and take into account postage, and possible returns cost.
Buying without auditioning is a risky business and gathering as much information as possible is always required. Be cautious even on sites like this, as there are brand lovers/haters and FOTM cans, as well as people talking "broadly" such as myself whos experiecnce with the Ultrasones was limited to under an hour with each in a shop.
I can clearly comment on my experience with the Beyer DT770, but my taste may not be the same as yours. I do listen to LOTS of trance/dance music and have no complaints with my investment. Again, good luck shopping.
