Just a quick post to draw on the collective wisdom of the crowd here-- I purchased both the 990 pro and the 990 premium 600 ohm, wanted to try out the different ohm rating. I also grabbed an e17 to drive them. Have been doing some A/B testing running out of the headphone jack on my x-fi platinum to the e17, using flac files. At max volume on the e17 (gain at 0), the 600 ohm are loud enough and seem to have more clearly pronounced bass, but the sound isn't overwhelmingly loud. The 250 have a more present bass, but it doesn't seem to be as clearly defined as on the 600 (clamping force difference?). Turning up the gain on e17 helps sharpen the bass impact and detail on the 600.
Return window is about to close, and the price difference between the two was fairly negligible. So my question is-- how much will a lower-end (sub-$200) desktop amp affect the sound on the 600? The 250 seem a bit bloatier on the bass, which at times I can mistake for a fuller sound, but it also seems like boom for the sake of boom. Finally, any good suggestions for test tracks?