aaDee
500+ Head-Fier
I had similar experience with my Takstar Pro80. Comfort is outstanding, soundstage is improved overall clarity also better but NO BASS at all. I didn't notice the change in bass at first but after buying Fiio x3ii, I was cribbing about X3ii' bass. Whereas other members were quite happy with the Fiio's bass.So I went ahead and ordered black HM5 velour pads for my ATH M40x headphones and it turned out to be a waste of money... Where as the comfort is outstanding and you get a noticeable improvement in sound stage there is a huge drop of bass frequencies. I used my Beyerdynamic DT811 (modified for flatter FR response) as a reference. With the stock pads the M40x sound great through out the frequency range allthough the have noticeably more bass than my Beyers. With the HM5 pads the bass goes way below my Beyers and they suddenly sound more like my dads vintage Pioneer SE-205 that has virtually no extension to either end.
There was a guy on Head-Fi who did a review of the HM5 ear pads on several different headphones and his conlusion of M40x with HM5 pads was that there is roughly a 6dB drop of low frequencies and the sound is unacceptable. I can't find the review now but I really do agree.
What did yield good results though was using a pair of DT250 earpads on the M40x. The change in sound is much more subtle but pretty much only for the better. There is a slight drop of mid bass and the whole sound signature sounds more open and natural.
For a change I tried using the original pads and bass came alive. I live in India and its always humid here. Can't use plether pads for long. I hope hybrid pads solve this issue.