I've had some success with the OEHLBACH mini-to-mini 120 Ohm adapter that Jan Meier offers. I bought it along with an HD600 for use with my favored V6's, and I can report that the accessory helps to buff down the highs (not chop, not whittle, not sand, just buff) and enrichens the midrange noticeably. However, it also gives an apparent increase in the upper bass, making the bass sound a bit warmer and thicker. I wouldn't pay full price with shipping by itself (which could amount to 25 dollars), but if you are ordering something else, Jan will sell it to you for only 5 dollars, which I think is a more than reasonable price for this tweak.