You don't need a pre-amp. You don't even really need a DAC, at least to start with.
Amplifiers can offer several advantages over, say, the output of your iPhone.
1) More power. This simply means the headphones get sufficiently loud and there is no distortion from clipping (amplifier running out of power)
2) Lower output impedance. This means that there is minimal frequency response variation with lower impedance headphones.
3) Other major flaws with output (not really applicable to iPhone), including but not limited to high distortion, audible noise or inaccurate frequency response - this is assuming the problems are due to the headphone amplifier in your source - if they are still present in the line out then the problems persist even with an external amp.
To drive the HD 650s you really don't need that much power relative to other "audiophile" headphones (sensitivity of 97db/mw, not to be confused with db/V), and their high impedance means the output impedance of your amplifier isn't much of an issue. An excellent amplifier choice would be the Fiio E9. It uses the TPA6120 specialised headphone amp IC, which performs superbly into high impedance loads. Even better, it can be found on Amazon and, should you find your computer's sound wanting, can be used with the Fiio E7 DAC/amp, which docks into the E9 to provide DAC functionality.
Both of these combined are considerably cheaper than $200-300 and would be a vastly better buy than the Asgard.
