With 300Ohm headphones such as the 600 (I have the HD-650 and the PXC-250), the question becomes important. The most affected aspect is dynamic range. Even my Onky805, which has an OK headphone amp, does not do full justice to the music, but I am still debating internally which headphone amp I will end up using.
For my iPod, both the HD650 and the PXC250 are ridiculously underpowered (you need to have the iPod at max volume and everything sounds like it was a the same loudness). Before investing too much, I bought a small gizmo
http://www.boostaroo.com/store_detail.php4?id=5 and the combination iPod+PX250+boosteroo is heavenly (great details and now complete dynamics). I will likely stay with this setup, although I am a bit annoyed that they are using AAAA batteries instead of AAA for which there exist chargers.
Combinarion Laptop+HD650+boosteroo is very good too. The combination Desktop+Audiobytes+boosteroo is OK, but not worth the change.
The combination Onky805+HD650+boosteroo is unfortunately terrible (really, REALLY bad; I think the little gadget does not like when the source is more powerful than an iPod or laptop) so I will be still on the lookout for a HT headphone amp.
