From owning the Sig Pro and auditioning the DJ for about 10 days, I'd add -
1. improved headband comfort
2. improved pad comfort/inner diameter larger
3. shorter cord option or better yet a standard 3.5 plug receptacle so you can have as short as you want. I use portable phones for working out/running/yardwork and put them in a holder on my upper arm and want only about a 22" cord, which I have had commissioned a couple of times but can't readily use them for the Sig series. Could for the Pro900 tho............
To me the comfort was bad. Really bad after about 30 minutes or so and had to take them off. Too bad since the sound sig was darn good on both.
I'm surprised to read your discomfort with the Signature DJ's - personally for me these cans are one of the most comfortable I own - I really appreciate the Ethiopian soft leather on headband and earcups - pure luxury that as far as I know only Ultrasone use on headphones, maybe they've cornered the market in buying all the specific breed of sheepskin that comes out of Ethiopia, lol - but I guess everybody's head is different in size and shape that affects level of comfort differently.