I recently became fed up with my PC 350 because of the comfort and SQ issues which made them unpleasant. However, I just tried this mod out and all I can say is "wow". I only drilled one hole on each side and after a first listening, what most struck me was the increase in bass. Music actually sounds alive, rather than snoozing in the back of the phone. I also noticed a considerable expansion of the soundstage and a general improvement in the mid-range. After popping on some of the velour pads from my 555's, the PC 350 are finally listenable again. Thanks so much ourfpshero, you saved me $150!
Good to hear that the HD 555 velour pads fit also, I use the HD 595 pads.