I own both and have compared them for many hours. I definitely prefer the 600 to the 595. The 595 take you a bit closer to the music, but the sound is colored. It seems that there is some cheap plastic vibrating and distorting the music to some degree. I can not recommend the 595. The grado SR 325, Sennheiser HD-600 and Stax 3030 have a cleaner sound.
An amp does not improve the sound quality that much for normal sources. Even without an amp the HD 600 sound cleaner than the 595.
__________________
Gilmore Lite with Lambdapower linear power supply, Porta Corda II, Terratec Sky or Archos 505 -> Stax 3030, Modded AKG-K340, Senn HD-600, Grado SR-325, Senn HD-595, Yahamah HP1, PX100, Senn IE3
Senn HD-650
Senn HD-600
Senn HD-580 Jubilee (to me a better phones than HD600)
Senn HD-580
Sony MDR-CD3000
Sony MDR-CD1700
Beyer DT-880
Ety ER-4s/p (excellent details but failed in comfort issues)
Grado SR-325
Grado SR-225
Audio Technica AT-A100ti
Under $100:
Sony MDR-V6
Sony MDR-E888
Grado SR-80
Shure E2C
Koss KSC-35 (great sound but cheap built quality)
JVC HA-D990 (Excelltent phones in this range)
__________________ Source: Modwright with Kern Modded Sony SCD-777ES w/ Tube Rectified PSU http://www.positive-feedback.com/Issue14/modwright.htm, Modded Sony SCD-1 Headphones: Sony MDR-R10,, Audio Technica ATH-L3000, W2002 w/ L3000 drivers, W2002, and Sennheiser HD-650 w/ Zu Mobius or Moon-Audio Silver Dragon or Stefan Audio Equinox Amp:Upgraded Single Power SDS w/ Blackgates, High Output Capacitance, High Voltage Option w/ Adjustable Bias (Stainless Steel Chassis.) & Audio Technica DHA-3000, HA2002 (best amp for W2002!!) Speakers:Paradigm Reference 100 V.3with Sony N/E/P9000ES Combo
I would say the Alessandro MS-2 for 300 american dollars is a great choice. most people say that the 225's are the sweet spot (best sound for the money) in the line, but i would have to disagree. i know it's not technically a grado, but it's close enough. anyways i think they are worth the extra money over the 225's (which are still a great choice mind you). You get much neater (and deeper) bass, and the highs don't induce headaches. plus they look cooler i think haha.
so if you were trying to decided between the 225's and the MS-2's like i was a while ago, I say save up your money a little longer and get the MS-2s. In my opinion, these are a definite canidate for one of the best pairs of headphones under 500.