I bought the sennheiser hd 280 pro about 6 months ago and I'm really happy with it. the sound is very good and the frequency response is pretty flat I find (I'm no expert ), there's no boost on the low frequency or anything like that
I have some Sennheiser HD headphones but I like my $100 Sony MDR7506 headphones a lot more. It just comes down to personal opinion but the Sony 7506's let me hear every detail so clearly, that's why I love em.
Finally i've bought a pair of dt990, i've tested for an hour both dt880 and dt990..
The dt880 have more balanced sound and for me they are also bit more detailed. However i bought the dt990 because this one have more bass, (in my track i overpower too much the bass so this can be an helpful thing when i'm producing.. At the moment the dt990 is great for me, next time i can look for choosing another level of headphones..