If you have a capable headphone amplifier, like the Axe-FX, do not choose your headphones based on its impedance! (unless you want to use them with a bad quality low-power amplifier, like a cellular phone)
thes are very good for the $$Massdrop has a deal with Sennheiser. If you're looking for a pair of HD-600s, consider either of these (sonically they're identical to the HD-600s).
Massdrop x Sennheiser HD 6XX Headphones
Massdrop x Sennheiser HD 58X Jubilee Headphones
(Yes, shameless referring link, but hey, it'll save you another $10 off your order if you decide to grab a pair!)