First of all, hello Indra and welcome to Head-Case!
I would say that there is a big difference in sound signature/presentation between the 2500 and 750, so that assumption seems wrong to me. And from something that I admittedly heard second or third-hand, the 780 isn't really an Ed.9, Jr. I thought Vicki was able to hear them in comparison, so maybe she will pipe up unless I am mis-remembering. [i just found her comments in this thread, so there you go.] If that is correct, then it would not seem like a good move from 750 even as an interim to the Ed.9.
Given that I own balanced Ed.9s, I would also disagree with the comment that they do not improve over the s/e version. I had my stock cable re-terminated with XLRs, and even that was a big change. It has more controlled and better defined bass and the high end gets more room to shine that way. At the MOA meet, a number of people commented that the balanced 9s on my SDS-XLR were the best they had heard those cans.
Just this past Sunday I was at Mikeymad's with JP#s for a few hours and we had Mikey's Rudistor RP100 and RSA Apache. The 9s sounded extremely good on the Rudi amp and really mated as well as Rudi and Ultrasone have made out. That is a very nice match. The 9s did not sound as good on the Apache. Just didn't drive them quite right, and I tried all 3 gain settings. JP and Mikey both commented on how much better the 9s sounded than other times they heard them, and in JP's case owned them. Interestingly, we found exactly the opposite synergy with balanced Equinox HD650s. They were muddy and dull on the Rudi and great on the Apache. Ray clearly voices for his favorite, the HD600, so that is not surprising.
If you have a balanced amp, re-terminating is a great option for the Ed.9, IMO. I am considering a re-cable at some point but I have not heard much one way or the other on whether it is a good change or who has done what.