The general consensus around here is that 990/250 uses the same drivers as 770/250. And while 770/250 do sound bit different from 770/80, they are not THAT much different (770/80's are bit less bright).
The following is what I found with my Headphile modded 770/80's (and what I wrote in one of the meet impression threads), which has screened woody, making them open cans.
My modded 770/80's sound completely different from stock 770/80's. Their sound signature is completely different from the stock and in a good way, IMO. I had a chance to compare my modded pair to a stock pair at the recent Boston meet and the difference was night and day. With the stock pair, all I heard was mushy bass and rather thin mids and highs. On my modded pair, the sound was much better balanced overall and mids/highs were more full sounding. As Wayne has put it, after he listened to my pair, they've become more "midrage" cans. I do remember Larry mentioning to me at one point that Beyers seem to benefit a lot more from recable than most other cans he's worked on.