I'll have a look at SoundMagic. What confuses me about Beyerdynamic is, well you recommended the 660s, what about the 770s, 880s, 990s? They all cost roughly the same. And why do they have models with different output impedance? Also forgot to mention, I don't care if they're open or closed headphones, I will be using them only at my PC
Then go open (smoother frequency response, bigger soundstage, often more comfortable)
Easy to drive :
-Sennheiser HD598: Neutralish, slightly warm and very smooth sounding headphone with big soundstage.
-Audio Technica AD900X: Neutralish, brighter than HD598 and very nice for female vocals.
-Philips Fidelio X1: Darkish, big mid-bass and soft treble, still a balanced headphone. Built with very nice materials, although not replaceable ear-pads.
-Philips Fidelio X2: Newer version, often over 200 dollars. Similar to X1 but more neutral. Still big bass, but more lively treble.
-SoundMagic HP-200: Sort of slightly v-shaped frequency response, with boosted mid-bass and raised treble, but still natural sounding and smooth.
Quite hard too drive :
(ie, DGX will drive them but an external amplifier would be a nice addition)
-Beyerdynamic DT880 250 Ohm : Very neutral headphone, slightly bright (coldish for some).
This is a very nice reference headphone. It's good with some metal but can be quite unforgiving with some modern recordings, sure it will make some of your recordings sound flawed (over bright, bass-light, even clipped). It's a revealing headphone so there's a trade off...
-Beyerdynamic DT990 250 Ohm : Sort of DT880 with boosted mid-bass and raised treble (yes, even brighter than DT880). Also bigger soundstage. It's a very nice headphone. Not great for bright recordings.
-AKG K612 Pro : A slightly warm headphone, but still very neutral. It's more forward in the upper midrange than the others. Some find them more energetic and lively, others find them more shouty and prefer a more relaxed presentation. Overall a very well rounded headphone.
Sennheiser HD600/650 are probably just a tad over your budget.
Hope this helps!