Some analysis for HD6** series in preparation for HD660. HD650 was purchased one year ago and KISS modded (http://www.superbestaudiofriends.org/index.php?threads/hd650-kiss-mod.3440/) HD600 is the dark mesh baffle version from a while ago Will more complete HD6XX / HD650 measurements a bit later HD660S measurements start here: http://www.superbestaudiofriends.org/index.php?threads/sennheiser-hd6-analysis.5308/#post-172286 BLUE = HD600 ORANGE = HD650KISS LT GREY = HD650 (historical data from 2016) Using same set of pads. As we can see, frequency response differences are not significant. Pad wear, condition of foam, and how we wear the headphones are likely to play just as much a role. HD600 is brigher than the rest. The KISS is darker with marginally less sub-bass. The latest version of ARTA will show distortion in percentage. HD600 distortion HD650KISS distortion HD650 stock distortion (historical data from 2016) Interesting. The overall bass distortion on the HD600 is lower than on the HD650! (HD650 does have lower third order below 40Hz). Is this why folks refer to the HD650 as being more veiled or having lower quality bass compared to the HD600?