I agree with you that Beyerdynamic products in general have a very nice sound to them. They tune most of their headphones to the old “diffuse field” target response. If you’re interested in going down a Google rabbit hole, there’s a lot of good stuff to read about that old target curve out there online.
Most modern target responses, like the Harman response, use more accurate and precise measurements than the diffuse field system. With the older DF curve, they were trying to mimic the sound of speakers inside a diffuse chamber, in an attempt to give the sound some live room-like characteristics. Newer headphone tunings are less concerned with the sound of a room and more concerned with raw performance.
I feel like the 880 is probably the “best-sounding” Beyerdynamic headphone I’ve ever used, and that there’s a reason why it holds its value so well in spite of being such an old product.
Shameless link to my review:
Beyerdynamic DT 880 Edition 600 Ohm Review
In my Sennheiser HD 58X review, I said I didn't know of any equal or better-sounding headphones you could get in the…
I think it has a sound that rivals the quality of many other “flat-response” headphones out there, like the Sennheiser HD 600 series. It’s an exceptional headphone for mixing, mastering, and critical listening.
The DT770 is a bit more v-shaped, and has exceptional isolation. Those are both things I really enjoy, so it’s probably my personal favorite Beyerdynamic product.
The 990 is a fun, v-shaped monster that won’t be to everyone’s taste. If you’re needing to invest a ton of extra money just to get the 600 ohm version and the equipment to power it, it’s probably not worth the push, in my opinion. It’s going to be just a tiny bit smoother in the treble, but it’ll still have the same basic diffuse-field-style sound signature. Soundstage is hard to quantify and won’t be that much different, and overall loudness/volume is dependent much more on your amplifier and the sensitivity rating of the headphones.
In fact, with the same amplifier, it’ll be easier to power the 250 ohm version to high volumes than the 600 ohm version, usually.
The 770, 880, and 990 are all based around the same core driver system and tuning. They use different coil materials, padding, cup designs, and dampening to achieve their different characteristics. The difference between an 880 and a 990 is far bigger than the difference between the ohm models.
I don’t think Beyerdynamic ever meant for people to overanalyze the differences between the different ohm versions, hahaha. It’s really just so you can pick the one that best matches your current equipment and its power handling capability.