I have a KR-6030 from around that era. There's some impedance mismatch between its headphone output (probably > 100 ohm output impedance) and my 250 ohm DT880, so it hisses and loses a bit of bass control. Despite this, I still think it sounds perfectly pleasant, with slightly rolled off treble and a tiny bit of extra bloom in the midbass. I wouldn't quite call it synergy, since it's likely a result of an electrical mismatch rather than any intrinsic coloration to the amp itself, but it's still not a bad-sounding combination.
Edited by dyang - 12/15/13 at 12:50am