It might have to do with burn-in and definitely also convenience in my case. I've had the H6 for about a year now and remember using the amp at first and then deciding to leave the amp at home (I'm using them at work mainly). The amp definitely added more volume and bass to the sound, but after...

  I'm using an iFi iCAN nano with my Sennheiser HD 600 connected to a MacBook Pro. But with the H6 I found myself connecting it directly to the MBP, rather than through the amp. The MacBook has a terrific DAC and enough power for the H6.