As stated above the Senn 600/650 are great headphones but require external amp because of 300 ohm rating. Headphones that are run directly from portable CDP/MP3/Computer need to be very efficent 30-60 ohm range, any Grado headphone would work also ideally get used 225 or 325 models.
To substantly upgrade sound for any headphone you need external amp to boost signal from portable CDP/MP3/computer, also need mini/dual RCA cable to connect headphone amp, ideally device has "line out" mini to directly connect headphone amp (eliminates cheap volume control of MP3 player)
Popular headphone amps for @$300 widely available include Creek, Musical Fidelity, Grado, Headroom. Go here to look at some headphone gear and explore options:
http://www.headphone.com/layout.php
Also to read top headphone discussion forum check out Head-Fi:
http://www5.head-fi.org
To substantly upgrade sound for any headphone you need external amp to boost signal from portable CDP/MP3/computer, also need mini/dual RCA cable to connect headphone amp, ideally device has "line out" mini to directly connect headphone amp (eliminates cheap volume control of MP3 player)
Popular headphone amps for @$300 widely available include Creek, Musical Fidelity, Grado, Headroom. Go here to look at some headphone gear and explore options:
http://www.headphone.com/layout.php
Also to read top headphone discussion forum check out Head-Fi:
http://www5.head-fi.org