It's time I chimed in on this mod, it is a must do for all E2 owners. I did this a few months ago, perhaps foolishly I did it the day after I got my E2c, so I don't have the most accurate impression of the difference it made, but I definitely did notice a big improvement in treble.
Now, months later, I've added an amp and got the ultrasoft tips, each of which make a big difference to my ears, and these sound fantastic! I don't EQ them, and I have no wish for any more for higher treble at all. Adding an amp really is a must for these phones, and your amp must be able to drive something as low impedance as 16 ohms. My solution is to use a cmoy, with resistors in series with the output, to bump up the impedance of the headphones, to make it easier to supply sufficient current. Bumping up the impedance in this way made a very significant difference to the sound, especially in bass depth and quantity.
Now I can listen to my E2c's very happily. My main cans are MS-1's, and I don't feel disappointed with the sound of the E2c's in comparison to them. I find the treble to be just as satisfactory, and while the bass on the E2c's is nowhere near as deep or plentiful, it sounds fine for most music and is surprisingly punchy. I would also say that when modded, the E2c's have a similar sort of sound presentation to the MS-1's, although they sound "littler", they have that nice lush mid-range, great for rock!
Overall a great mod, anyone who hasn't done this, please do!