Tonight I was bored so I decided to open up a couple of cheap headphones I had bought a long time ago. I opened up a pair of Sony MDR-CD180 and a set of RadioShack Pro-35's (aka Koss KTX-Pro). Then for some dumb reason an idea struck: why not put the KTX-Pro drivers in the CD180 housing? I performed the surgery and the results aren't too awful. The KTX-Pro drivers seem to have more treble and a less boomy mid-bass than before, but this set has a lot of echoes. Still quite an improvement over the standard Sony drivers. They are loads more comfortable than regular KTX-Pro's. I will try adding some cotton or something to the inside of the earcups and see if that helps absord some echoes.

Would it make a difference with the PortaPro or KSC50 drivers instead? I know they are all supposed to be the same, but the differences in each are so apparent that there's got to be a physical difference.

I don't think it should make any difference, but I haven't been willing to open up my KSC-50's yet to see if there is a difference. The main problem is the rather large enclosure of the CD180's and the way it is designed. It causes a lot of echoes and the way the drivers fit on the Sony's leave extra air vents for backwaves from the driver to go back into the ear. The way the Sony drivers are mounted causes those air vents to be closed. I am trying to figure out a way to really close those air vents so that only the front of the driver actually directly creates the sound waves just like the original Sony drivers. The enclosure seems to be big enough for backwaves to have enough room so they can move as much as they need without interfering with the operation of the driver too much (kind of an inifinite baffle I guess).