I recently purchased a pair of Beyerdynamic DT 990 premium (32 Ohm version) headphones and I was quite let down, mainly because of rather tinny symbols and a unbalanced sound signature (wherein the weak sounding symbols and drums drown out everything else). I should mention that I used these exclusively with my iPod Classic, unamped. All this being said I have found a buyer and plan to sell them.
With the funds from this, I was debating between the Beyerdynamic dt 880 premium (600 Ohm version) and the Sennheiser HD 600. I can purchase the former locally used for about 200$ to 225$ total, and the latter new online for about 350$ total. I also plan to use either pair with an Objective2 Amplifier. I was wondering what people would recommend, noting my disappointment with the tinny symbols and how predominant they were, and the amount I can secure these other headphones for. I should mention that the new headphone will be used exclusively for listening to music, via the O2 and my iPod Classic with loseless files.