I don't quite get it. The Senns have very similar sonic signature (with relatively slight differences in frequency responses). What you are looking for can be more effectively (and cheaply) obtained by EQ-ing the bass regions.
If the hd600 is somewhat like the hd580 (which it should be) then there is a definite difference between the 600 and 650. The 580 I have is NOTICEABLY brighter than my 650. It has a distinct treble spike which the 650 lacks.