Compared in balanced mode on HM-801, the 600 are slightly superior to the 272, yet in the same league.
272 are more dry and analytical with some snap while the 600 are more organic, more rounded so to speak.
The 600 have fuller bass that make it sound more bold. The sound is definitely more fluid, sweeter, and natural, although not as extended in the highs, not as airy up top.
It's all very subtle though. They share a similar overall tonality.
To me, the 600 are a mix between the 272 analytical and the 262 organic signature with the added fluidity/sweetness of the 400 and a tad more boldness in the bass department.
I was a little disappointed at first because I thought they sounded very much like my 272. But after over 200 hrs of burning, I can now hear the difference. I hear why they are advertised as Hifiman's best IEMs to date. 600 sound a little warmer, more analogue and fluid, more musical and natural.
I still highly regard the 272 for its overall balance, tuning and extension. Properly EQed, the 272 sub bass and highs quality are very impressive to me. But we're talking extremities here.
600 beat everything in between.
For most music styles, the 600 sounds more natural and authentic. For electronic music, some may prefer the 272 (I like them both)
A version of the 600 with the 272 sub bass and extended airy highs would be a killer IEM, maybe the Re-6000...
In the end, I was going to sell my 272 but decided to sell my 262 and 400 instead ; )
Edited by Tympan - 10/18/13 at 8:39pm