This is what prompted me to buy a pair of new k601 last week. It seems to be the consensus that they are a bit darker, less forward and slightly warmer and bassier. I found this to be true. However I did notice they were a bit congested and seemed to blend instruments sometimes.
I decided to switch the pads from my 702 over to my 601. Immediately the soundstage opened up to a level similar to the 702. Separation also improved immensely. I'm not sure how but bass frequencies also seemed to be accentuated, perhaps due to better frequency separation. The result is a better perceived frequency response from bass - treble. A huge improvement to say the least.
I also decided to put the k601 pads onto my 702 just for kicks. I could hardly believe what Iwas hearing. Suddenly the music was closer and more intimate but with a much more balanced frequency response. I'm hearing sub bass notes that won't be heard with traditional 702 pads. The midrange also became more even with the rest of the spectrum. Midbass is elevated slighty, enough to be more of a "full" sounding headphone.
I guess this shouldn't be a huge surprise considering these headphones are almost opposites from each other and have been outfitted with pads that take them even further away from each other. Dark, enclosed vs bright and open. Though I think both go too far and become too unbalanced. The 702 pads, being thicker towards the back create too much space between the driver and ear. This causes low frequencies to die out before reaching the ear. The 601 pads are smaller and completely flat which brings the driver closer and helps midbass but restricts soundstage and separation.
Using the right pad for the right headphone is paramount. I would go as far as saying that pads are 30% of the sound signature of any headphone. This is similar to speakers where the enclosure and porting determines the end result. Of course this is also highly driver dependent. The 601 driver is naturally darker and bassier. The 702 driver is brighter and a bit more clinical. Using pads that complement and help control each driver is important.
I'm listening to my 702 and enjoying it like never before.
Edited by Punnisher - 6/13/13 at 5:26pm