Honestly, the PX-100 doesn't stand a chance. The DTX501p is better in every aspect - except in terms of comfort, because I can wear the PX-100 all day long without realizing it. The Beyers bass is significantly more accurate. The mids are better and not recessed. The PX-100 is nice for jogging or as backup cans, but you hear the price difference between the Senn and the Beyer.
I wouldn't be upset if you also got the JVC HA-S500 ($70) and compared it to the DTX501p for me :)