Are you in the recording industry?
You are talking about the Fostex T20s yes? The drivers in those are planar magnetic instead of the regular cone and round magnet dynamic style.
The mod fine tunes the sound to make them more enjoyable as an every day headphone instead of a studio reference headphone. The Fostex T20/40/50 are one of the best bang for your buck user modable headphones out there, you can actually fine tune the sound (within reason) to your liking. Planar magnetic drivers are fast, but do require a bit of power behind them from an amplifier to really shine.