I'd look into the Aune T1; I use one for my HD558's and love it.  At the moment they're $160 on Amazon, $30 less than I paid 3 months ago.   There are some good threads on it here.   http://www.head-fi.org/t/633006/aune-t1-usb-tube-dac-amp-discussion-thread-see-first-post-for-faq...

I recommend checking out the Sony MDR-V6.  Very good soundstage, Great bass, and great as an all-rounder.  No amp required.  I prefer them over my HD558's (foam modded) for everything I listen to except Jazz.   I would definitely NOT recommend the ATH-M50's for ANYTHING, but especially not...

Fusion-era Miles Davis  

My test albums:   Dire Straits - 1978 debut Steely Dan - Aja Steely Dan - Gaucho R.E.M. - Murmur (mfsl)