The adapter contains 2 resistors (of varying resistance/impedance, common is 75 ohms) that is soldered into the left and right channels of the headphone cable. It just adds impedance/resistance into a headphone.
Example: ER-2P (canal phone) is an ER-2S (more expensive canal phone) minus 75ohms of impedance.
you could easily make the ER-2P to a ER-2S with just a 75ohm adapter...
(Lesson: additional impedance makes some headphones sound better (and more expensive), but harder to drive)
For your problems, You'll want something full size, since it's much more comfortable for long use. Sony's MDR-V6 (discontinued, $60 in some shops where available) and the DT770 Pro-80ohm ($150+ brand new) are much like the kosses in term that they have good bass and highs and the weak mids problem.
Note: if you like the PP2's sound, you could invest on it's full size version, the UR40 (w/c is also as cheap as the PP2, can be bought for less than $40 IIRC). Then sell off the PP2....
PS: Hell, I amped the KSC55s a minute ago, plus the 75ohm adapter. source is an iPod (headphone out, think if i used the line-out).
i think i'll never be happy again with most of my equipment(the PC, the ipod and my Compact-CDP by itself).. the bass was thumping and the highs were just right(not painful, not weak).. The mids were much better since the volume helped them a bit (IMO/for me)....
although the amp did a bit of low noise hum (meaning bad circuitry), but it was ok since it was barely audible at idle, and invisible when the audio starts (think if i used a better amp)...