My vote goes to Sennheiser HD-280 Pro's. I tried all the usual suspects - Sony MDR-7506, Beyer DT100's and the AKG 240 Studio versions before deciding between two - the Sennheisers and some Audio Technica HD40's. Tough choice, but I went with the Sennheisers.
Are you definately sure it's the headphones distorting, or is it the amp? If the headphones are high impedance, the volume will be lower, and as you turn it up, the amp can work too hard and distort.
What amp is it?
Ta
Dodge
P.S. I'm not a Mac man either, but 4.5 GB of RAM should be plenty for any audio application.