That’s surprising, I’ve actually owned a lot of this stuff at some point or another. Anyways,
- Audio Technica ATH-AD700X - Huge soundstage, it can almost sound like you are standing in the game/video/music. Downside is that the bass is underwhelming and the treble can be sharp if you’re not used to bright headphones.
- Beyerdynamic DT990 - Moderate soundstage. The frequency response is more of a ‘V’ shape, there is an emphasis on bass and treble on these. Probably the most “fun” headphone out of the choices, but not for everybody. I would scratch the 250 ohm version off the list entirely since the 600 ohm version is cheaper.
- Sennheiser HD 558 - Moderate soundstage. The frequency response is more reserved than the DT990s, but still has some emphasis on the bass and treble. These would probably sound good to more people than the DT990s. Not too hard to drive either.
- Sennheiser HD 6XX - Effectively rebadged HD 650s, which are easily $300+ headphones. Of the headphones on the list these are the most balanced headphones in my opinion, the soundstage is very good and the frequency response is neutral with just a bit of excitement. If you want what you are listening to to sound nearly exactly how it was recorded these would be the best bet on the list.
My personal order of preference would be
- Sennheiser HD 6XX
- Sennheiser HD 558
- Beyerdynamic DT 990 600 Ohm
- Audio Technica ATH-AD700x
As for the amp/DAC, yes, the Mayflower O2+ODAC will drive 600 Ohm headphones without a problem. However, I do think the Schiit Modi and Schiit Magni would be a better combination nowadays. There is nothing wrong with the O2+ODAC, but it is a bit dated now since newer audio ICs have come out. The Schiit Modi+Magni, affectionately referred to as the Schiit Stack, has more power capability while having same or less distortion and noise.