Ignore the trolling by "techboy" (note that a substantial percentage of this guy's posts is bashing ASUS cards in a large number of threads, even cards and with headphones he has never heard), if the Phoebus has the same amplifier as the STX, then it should not have much problem driving the HD600. Of course, you will need to set a higher gain than with the ATH-M50, but it should be fine otherwise. Do not buy worse headphones with an amp. The only problem is you may possibly prefer other headphones, so it would be best to try some before deciding. The HD600 is good if you are looking for a neutral sound, and do not need a lot of bass.
xonar phoebus - Page 2
Gear mentioned in this thread:
Get the Asus Xonar DX or D1 sound card, used $50-$60, same CMI8788 audio processor as the Essence STX/ & ST, Phoebus uses the CMI8888.
I believe the Schiit Magni, $99, amplifier is a little more desirable then the Essence STX/Phoebus head amp.
Oops, missed that part about you already owning the Phoebus.
Did you turn off the motherboard's on-board audio (in the BIOS) when you installed the Phoebus?
Try using Foobar200 for playing music audio.