I use the PA2V2 (my only portable amp) with either a Coby MP823, or a Samsung Corby-Pro phone. File format is MP3 (some 320 kbps, some less) and WMA (medium-high quality). I know, pretty embarrassing.
Normally I would set the source's volume to 70% when listening to easier cans (HD-418, MDR-V6). With the HD-600 I do have to crank the source up to 100%.
I've played with the gain screws in the PA2V2 but eventually turned them back down to factory-set level, maybe slightly higher. Around 1 or 2 o'clock on the knob gives me a comfortable volume.
A couple hypotheses on why you're not getting good results:
- You may be wanting more volume than is possible with this amp. I like to be able to listen to music for hours and hours without tiring my 45-year-old ears... Then again, I sometimes go loud if the music demands it (e.g. Verdi Requiem or Led Zeppelin) so I have pushed it to my own ear's limits without any issues. But you don't have my ears, so...
- Batteries. I know the LED is supposed to tell you when they're dead, but distorsion may be a sign that they need a recharge. I experienced this once listening to Sheryl Crow at a very medium volume - there was a lot more Crow than Sheryl in those vocals, if you know what I mean. After a night's charge the problem was gone. Also, you are using 1.2V rechargeables, right?
I use freshly charged 1.2V NiMH. The amp will blow your brains out with the HD25's. Just not enough power to properly run the 600.