I just cant see how messing with the excellent pads on these headphones is a good idea.
i think it depends on what kind of sound you're looking to achieve. i think both the q701 and the ad 900 are great headphones, but they're both different. if you're looking to keep that audio technica sound, you want to go with the ad 900's. if you're looking for something with a lot better bass extension and much more accurate timbre, the q701's fit that description. the ad 900's essentially ad 700's with much better midrange IMO. q701's are just linear and accurate. i'm sure many people would find them boring.
Not necessary you will like the upper model. I like my AD700 more than AD900 that I tried in the shop.
In fact, I would say AD700 and AD900 is quite different in sound.
I agree that AD900 is more detail than 700, but I love 700 overall tonality balance and slight midrange warmth much more than detail.
I find 900 sound could be little tiring for long time movie watching session, but I never tried at home.