I've been looking for practice headphones myself recently.
I'm specifically looking for open-back or at least semi-open. I guess I what to hear my nails scratching the strings

I'd also prefer headphones that are be easy to drive. High impedance headphones like most AKG and Beyerdynamic are pretty much useless without good amp. My bass head didn't come equipped with one. Besides I prefer to use old Line 6 POD XT for silent practice, there is no headphone amp in it as well.
Here is what I've tried so far, hope you find it helpful:
AKG K701. I might have received damaged item. There was strong buzz in one of the ears on B or E string. Packed it back and returned within an hour. There is a lot of praise for AKGs but it looks like much of it is based on the older models made *not* in China. Nowadays it's like $400 headphone for sale at $150 everywhere. It's hardly worth that $150 IMHO. I'm not buying from them again.
Beyerdynamic DT 770 250 ohm. Sound is good and perhaps the best of what I've tried. But it's closed-back. Sound has always felt very distant, like if it's not you who is playing the bass. I haven't tried DT 880. Amp is must have.
Shure SRH440. Ok for practice, but closed-back. IMHO quality is ok for the money (~$100).
Fostex TR70. Hard to drive. There was nothing I liked about them. Sounded dull, it could have been due to lack of break in. At first there seemed to be annoying vibration within the headphone itself at specific place on the fretboard. It might have been fixed by another amp.. I've got used item with incomplete packaging sold as new. Returned within days.
Grado SR225e. You either like them or hate them for the music listening. There is nothing neutral about those headphones. Worked ok for practice except cable is too short, they are uncomfortable, rotating cups were driving we mad -- extremely unpractical.
ATH M50x are being recommended often, but frequency response doesn't look neutral:
http://www.innerfidelity.com/images/AKGK240Monitor.pdf
There are other options with great frequency response at least in bass section if you are ok with closed:
Akai MPC (also sold by M-Audio) for ~$180
http://www.innerfidelity.com/images/AkaiMPC.pdf
Tascam TH-02 (same as Teac CT-H02) for <$20
http://www.innerfidelity.com/images/TeacCTH02.pdf
I haven't tried neither of those.
For open-back I'm currently deliberating between trying Phillips SPH9500 ($70), DT 880 (>$200), HIFIMAN HE-400i ($400). Please share if there is anything else worth trying.
I'll most likely go with cheaper option.. from my experience $200 2x10 bass speaker does much better job at reproducing bass (at much higher volume) than $200 studio/monitor/younameit headphones.