The Beyer amp may or may not be sold separately. Also it is probably 500+ pounds(money).
The line in to your PC presents another gaming issue. The line jack only supports 2 channels. This translates into an improper decoding of Virtual 3d surround sound by your soundcard. Unless the PS3 has some sort of virtual decoder built in. Does it?
AFIAK virtual 3D sound works like this: The OS or game console is set to 5.1 or maybe even 7.1 (if the DAC can handle that) to the DAC. The DAC translates the signal into stereo and uses the DSP to emulate the 3D surround sound experience. I may not be 100% right in the actual implementation or terminology, but this is the basic idea, as I understand it. The DAC can be a soundcard (as soundcards have a DAC built in, DUH
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This is the ultimate gaming/movie experience. To me, it is SO much better than typical speaker surround sound setups.
Here is why I believe this to be so. Unless you, professionally, set-up and tune your speaker placement and output, the experience can vary INCREDIBLY. Also, if you do set up your surround sound it is centralized in one location. Therefore, if you want to chill on your bed instead of your chair and play some video games, you will either have to readjust your speakers or just deal with the imprecise sound.
So to have this ultimate gaming/movie experience, A DAC/Soundcard with a digital in (optical/coaxial) and virtual 3d sound processing is a must.
Considering your budget and what it seems like you want, you could purchase the AD700s, which are a steal and a great value, and invest the rest of your money in a DAC.
The DAC will be re-usable and part of your gaming config. Then, you can invest in better headphones or an AMP in the future.