I haven't tried using just the white foam and no T4000-filters as AstralStorm is preferring. Either way both mods should provide more treble I believe but in different ways. Perhaps removing the white foam will lead to treble peaks in some regions - I don't have measuring instrument to check this.
I have listened to dual T4000-filter/no foam-mod for many hours (lots of different music genres) and didn't find it strange sounding at all. Imo it works very good with all kinds of music and I like its sound character. To me the dual T4000/no foam-mod sounded like an unmodded RE-400 but with more treble presence. Anyway I have ordered another set of RE-400 so it will be nice to compare them directly.
Just looked up what the foam mod on older RE-0 does (should be the exact same for RE-400) and found the following (answer from ClieOS):
What exactly does removing the foam do?
Reduce acoustic impedance, cut down bass and increase treble.
I'm not exactly sure what acoustic impedance is but is has something to do with the speed. I find that the RE-400 copes really well with very high tempo (BPM) songs like in drum and bass. It retains all the detail exceptionally.
OK so if you remove the white foam you cut down bass and increase the treble, that is also what I found when I removed the white foam and used just one T4000-filter. So I liked the treble but not the bass it was too thin. Even if the treble was a little filtered out it was still more than with the white foam on. I wanted the bass back to the way it was from the beginning - that is why I put on a second layer of a T4000-filter.
I don't know why but there is a lot more bass presence when using two T4000-filters instead of just using one and because the white foam is removed there is also more treble presence. Also tried a third layer of T4000 two days ago and all it did was give more bass and a little less treble. So the more T4000-filters you use the less treble and the more bass, if the white foam is removed.
I went back to using two T4000-filters as I liked that sound signature / balance more.
Edited by Deni5 - 2/23/13 at 8:10pm