Yes, the TxxRPs are notoriously dark. In this case it's due to what is called "under damping" Basically the driver moves a bit too easily, making the big wide frequency low notes louder. So to help control the driver most people "damp" the driver. Usually by reducing air compliance. Meaning how easily air can escape or enter the driver and cup. So by taking out the solid plastic on the vents, and replacing with felt, you actually increased how easily air can move in or out. But it may be small enough you don't notice, if the felt is rather dense. The 2 biggest things you can do to help, is 1: fill the cup with rolled cotton (some use wool, cotton balls are ok if you have nothing else) and apply felt, or wool, or memory foam or something directly to the back of the driver (this single mod alone one arguably makes the biggest difference). Which reduces how easily air can move. If are careful not to damage the paper on the back of the driver, it's 100% reversable. A big thread on the forum is this one http://www.head-fi.org/t/618659/fostex-t50rp-incremental-mods-and-measurements In which lists most common mods, with measurements to see how the sound has changed. It is referring to the mk2 RPs, but the mk3 has the same driver, and identical cup. So most mods should apply; beit perhaps slightly different.