Thanks for that. Judging by what you and Mr Eleventy say it seems the differences are marginal. With a 250 ohm impedance you will still get a massive damping factor with the O2's low output impedance. I read one review on HTShack where the reviewer thought the 600 ohm might be intended more for tube amps to counter their high output impedance which isn't an issue with the O2. If I go for the 250 ohm now need to decide between the Pro and Premium (or "Edition" as it is described on Beyer's website). Can you confirm which one you have if you say it sounds great with the ODAC/O2. Is it the Pro with the curly cord or Premium with the straight cable?