Interesting, as I wasn't sure if people were going to try the O2 with the HD 598. How does the HD 598 sound at low volume with the O2 as opposed to no amp? I don't listen to music that loud and I'm wondering if the amp will have a real benefit.
One of the things that attracted me to the HD 598 was its low impedance, meaning it can sound good even without a separate headphone amp.
the 598s were my first set of hi-fi headphones and my only piece of good equipment at the time, so I can attest to their quality without an amp. With the proper tubes they have more detail, but I loved them from day 1. My iPhone powered them to sufficient volumes and my MacBook Pro can drive them louder than necessary.