Hi there!
I´m buying a pair of Beyerdynamics DT 770 PRO to couple with this portable amp:
 
http://cgi.ebay.com/Micro-Cmoy-headphone-amp-Opamp-2227-Amplifier-/260618937865?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3cae196609#ht_2524wt_1137
 
Which Impedance should I choose??
 
thanks a lot !!!

unless i'm mistaken, the pro model doesn't come in 600 ohms, only 80 and 250.
 
and, the general consensus is that the 80 ohm model has lots of bass, and the 250 ohm model has a little less bass.
 
also, generally speaking, the higher the ohms, the more refined the sound.  but the reasons why are a little complex...

 
I've been reading up on this model.   The big question seems to be bass and treble response.
 
What i have seen written often is to avoid the:
250 Ohm version pro or no-pro (usually called consumer edition here, and has two premium versions on the market).  250ohm usually said to miss on the bass and mucks up the treble.
 
Get the 80 ohm pro if you love bass.   Treble may still be there, but bass is in the lead on most tracks.
 
Get the 600 ohm custom built from beyer or 'premium' from B&H photo (or other vendor) for a balanced sound.   Hard to pin down a bass report on this one.  Some say it is flat or has no bass at all?  Then some jump in the thread and say all DT 770's are bass monsters and everything gets confused.
 
Does the DT 770 non-pro 600 ohm as seen at online retailers (cheapest, http://www.bhphotovideo.com/c/product/614489-REG/Beyerdynamic_491306_DT_770_Premium_Closed_Back.html) have good bass?
 
Is the DT 770 non-pro 600 ohm amped a good buy for random listening across most genre's or does it have a specific niche?