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1) Quarter Mod to the Pad = lets out more highs and mids, less muddled sound
2) Kramer Mod = UNLEASHES the treble, makes these things shine
3) Recable Mod (mini Star-Quad/ebay SPC seems to be the winners) = tightens up the sound, increase in overall clarity especially if you have a Koss cable with an in-line volume controller
4) 75 Ohm resistor mod (soldered in parallel) - tightens the bass so its less muddled/flabby
Think that about covers it. Overall, cheap upgrades that transform this into an amazing bang for the buck portable. (Quarter mod/Kramer mod free, recable ~10 bucks maybe (check navships SPC teflon on ebay), 75 ohm resistors can get it for cents @ Radio Shack or a few bucks for Allen Bradley types on ebay). Super easy to do, and if you mess up, you got Koss's No Questions Asked Warranty to cover you.
As long as you have a knife and soldering iron, your good. Very good to practice your DIY skills.