I run a pair of Sennheiser HD 558's that have lasted me a few years, and I still love them. They travel between home and work daily and get hours of use. They're comfortable on the ears (I used to get rashes from an older pair of cheaper headphones). They're open-back with impressive sound quality and a lovely even mix.
May be a tad over OP's budget, but not by much. It's pretty much the best model of Sennheisers that you're going to get in the 50 ohms range.