I have been switching back and forth between each type of the three Comply foam tips for the last two weeks to get a hold of how differently each is. So far I must say my personal favorite is still Comply T400 (in general), though it is not to say I dislike the others. They all have some pros and cons in comparison.
T400, being the original Comply, is what I am most comfortable with. It is soft, not irritating, and generally has the least impact on sound as compared to the other two. This means I can use it on most IEM without worrying too much of any sonic change, but I do expect the sound signature to turn just a little warmer, smoother and slightly less detail on the treble end. This is perhaps to be expected of using any foam tips and not unique to Comply.
TX400 pretty much does the same as T400, consider they are based on the same foam. However, TX400 also adds more bass weight and takes away just a little more treble because of the foam filter. It is not necessary a bad thing as I have found that TX400 works rather well with IEM that usually being described as analytical or leaner on bass (i.e. Head-Direct RE0, Phiaton PS210, etc). The added filter also works well on stopping earwax from getting to the IEM filter, which can be a pain to clean. This is of course great news for IEM users who tend to have more earwax. On a side note, I also prefer the black color as it is the least obvious when it becomes dirty.
To be fair, the blue colored S400 doesn’t quite look as impressive as its T/TX series counterparts on first sight. I was worrying the rougher outer surface might be irritating in use (as compared to the T series smooth surface), but I was wrong. It pretty much feels just the same as T400, and I do find it to have a little better grip in the ear canal than T series. One important thing I have observed on the S400 is the slower expansion after squeezing which gives it the ability to hold its shape much longer than T series after removal from the ear canal. It is a benefit when situation requires the user to do a quick remove and insert (i.e. try to listen to another person talking). The foam holds its shape and there is no need to squeeze the foam again. In comparison, T400/TX400 will usually just spring back to full size once removed and often needed to be re-squeezed for better fitting. In a sense, S400 does apply just a tiny bit less pressure to the ear canal. The only minor downside about S400 is that it tends to absorb more treble than T400/TX400, which could be the result of a narrower nozzle opening, the rougher surface which absorbs more sound, or perhaps combination of the two. This is probably the only reason why I personally prefer the more revealing (sonic wise) T400. Fitting and comfort wise, I think the new S series could easily equal or better than the T series depends on personal preference.
S series is supposed to be a different type of foam though the typical soft and tender feeling is just as obvious as other Comply. It is also going to be cheaper (US$9.95 a tri-pack) than the current T / TX series which should offer a better bang for the buck for foam tips user.