If you are happy with the sennheiser "house sound" you may want to just go higher up the line.
After many months research looking for a good headphone rig I settled on the senn 650's with a grace 901 amp to power it. This has proven to be a very good combination for my tastes. I will admit that at first I was not impressed with the 650's but after sufficient burn in time the cans now sound great!
If you are sticking to the sennheiser brand I would look at these amps (in no particular order)
Solid State
Heed Audio - Can Amp
Lehmann Audio - Black Cube Linear (Sennheiser used this amp to debut the 650's)
Graham Slee - Solo
Headamp - GS1 or Gilmore Lite (with dedicated power supply)
Naim - Headline (with NAPSC power supply)
Tube
Antique Sound Lab - MG HEAD DT OTL MKIII
Melos - SHA-1
Musical Fidelity - X can v3
Rudistor - RP5.1 (hybrid)
Woo Audio - Woo Audio 6
If you go in a different direction such as AKG or Grado, the amp selection will change somewhat.
Most of the amps listed will have to be purchased used in order to meet your $600 budget but thats all part of the fun isn't it?
Good luck on your hunt!
Dave
After many months research looking for a good headphone rig I settled on the senn 650's with a grace 901 amp to power it. This has proven to be a very good combination for my tastes. I will admit that at first I was not impressed with the 650's but after sufficient burn in time the cans now sound great!
If you are sticking to the sennheiser brand I would look at these amps (in no particular order)
Solid State
Heed Audio - Can Amp
Lehmann Audio - Black Cube Linear (Sennheiser used this amp to debut the 650's)
Graham Slee - Solo
Headamp - GS1 or Gilmore Lite (with dedicated power supply)
Naim - Headline (with NAPSC power supply)
Tube
Antique Sound Lab - MG HEAD DT OTL MKIII
Melos - SHA-1
Musical Fidelity - X can v3
Rudistor - RP5.1 (hybrid)
Woo Audio - Woo Audio 6
If you go in a different direction such as AKG or Grado, the amp selection will change somewhat.
Most of the amps listed will have to be purchased used in order to meet your $600 budget but thats all part of the fun isn't it?
Good luck on your hunt!
Dave