I just tried the TP mod on my S400s.
After doing the TP mod the lack of definition that I have been hearing on the mids in general, particularly annoying in voices, has gone. The mids remain a little bit recessed, but I can live with this. The highs have also fleshed out a little bit which makes them more natural sounding. Bass is still great, may be the mid bass is more prominent (but I am not sure). For the cost of a toilet paper piece and five minutes of my time I am immensely happy with the improvement I’ve got.
From what I hear, I believe that performing a mod that dampens resonances is a must for these headphones.