I've owned the SR325is, and I absolutely loved them. How they could be driven directly from my laptop only made me love them more. It's detailed, soundstage is mindblowing, and the bass is still good, which is frankly hard to do with open headphones. Beauty! My only regret is not getting my set...

A good option that is easily within your price range is the iBasso D4 Mamba. I feel that the best would for it to charge and play while plugged into your Macbook.

Electric Avenues PA2V2 is a small, cheap and good amp.

A good set would be the Sennheiser HD-1-25-ii. I personally use this set for on the go usage and it sounds fantastic. It can be driven directly from your iphone/ipod, but would benefit from an amp. Sound-wise, it's essentially neutral. Great clarity and bass depth has made it a huge favorite...