Have you tried pressing the cups against your ears to get an idea of how the sound will change as the pads flatten with time and use?
How thick are your 770 pads? If that pads aren't that worn, DT770 is quite incredibly bright, but if worn, it is more tame, not so over-the-top. Not much brighter than a stock HD25. Worst case scenario: your HD25 is defective.
I doubt this hd25s are defective. I think i found them a bit awkward sounding at first because of a group of factors..... First i have been using spacious sounding headphones for the last 2 months.... When you get something on ear after so much time using big over ears i think its normal for you to find the sound a bit boomy or cramped. Then i usually use rock eqs.... I was used to using my previous hd25s on my 4th gen ipod touch.... The rock eq on my ipod touch was v shaped but it made the sound tighter because itt was not too v shaped. This time with the new hd25s i tested them at first with my vodafone smart ultra 6, in wich i was also using a rock eq, but this v shaped eq in this case it is muuuuuuch more exagerated on the midbass and i think thats why i thought they sounded a bit boomy. I changed the eq on the smart ultra 6 to "normal" wich is a more slight v shaped eq and now it is much closer to the sound i heard on my ipod before (hd25s are super sensible to equing). Also i had tested them and let them burn with a first gen moto g and even with the eqs turned off, the moto g is too bassy sounding. So i trully believe eqs and sources are the issue here.
I stopped using my ipod since i got my premium subscription of spotify (cant have spotify on the ipod because of the outdated software). But to make sure i took my ipod out of the drawer, charged it, used the same music and eq i used with my previous hd25s and there it was.... The beautiful sound i was used too from my previous hd25s.
But as you stated, it can also bes because the pads are new. I still think they are a tiny tiny bit more bass heavy but the sound is still punchy and controlled and the highs extension i always loved os still there. Maybe they will get a bit better with use, maybe i will get more used to them again but i highly doubt they are a defective pair because they still sound great with the proper settings and sources.
About what you said of the dt770.... I have a love/hate relation with those. Got the, thinking they would be a roomier, more spacious sounding hd25 or m50x (everyone here raves about them being bassy or sucked out on the mids) and in the end i just found the to be nice sounding..... Spacious, yeah, well separated sounds and layers, but the hd25 and m50x are much more fun and punchy.
Edit: ....outdated drivers my ***....no other other headphone can make ilan bluestone's "Big Ben" sound as big, punchy, powerfull and clear and controlled at the same time as the hd25..... Goosebumps!!!
The m50x is pretty close though....