Check this out:  http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html

Thanks for the reply. I agree when you say HD800 is the closest you can get to a pair of monitors. As I said earlier I'd be using it for mixing and mostly for mixing Orchestral Scores. But I am not sure if I would be doing justice to a 300 Ω headphone to a 12 Ω amp of UAD Aollo. Would it be able...

I know it's an old post but I'm sailing in the same boat now. I own a UA Apollo Quad Firewire.   Output Impedance Line: 600 Ohms, Monitor: 600 Ohms, Headphones: 12 Ohms I'm planning on Sennhiser HD800   Nominal impedance: 300 Ω Would it work? Or I'd be stupid to make that...

I know it's an old post but I'm sailing in the same boat now. I own a UA Apollo Quad Firewire.   Output Impedance Line: 600 Ohms, Monitor: 600 Ohms, Headphones: 12 Ohms I'm planning on Sennhiser HD800   Nominal impedance: 300 Ω Would it work? Or I'd be stupid to make that...