Yes, those are the ones. They come in a number of different sizes, and are very handy for modding IEMs (or for even when the stock screen gets lost, damaged, or clogged with ear gunk).
You are right though - the nylon screens (such as the red ones KZ uses) - DO generally increase the sub-bass. But it is usually at the expense of clarity, better mids, and better treble. The nylon also usually adds a veil to the sound, and the bass it does add can often be tubby and/or bloated.
It is because of this that I think nozzle screens are a poor way to tune the bass/sub-bass of an IEM because of the negative way it affects the rest of the sound. A MUCH better solution is to use the stainless screens on the nozzle, and instead tune the bass/sub-bass by manipulating the size of the bass vent hole on the IEM shell. You can also fine tune it with different tips (usually with a smaller bore opening such as the KZ Starlines have).
I also usually save my screens when screen swapping on a new IEM - for example, if I remove the red screens from a new pair of KZs, I'll replace the screens with the stainless version. And where I peeled off the stainless screens from the plastic sheet, I will stick the red nylon ones in it's place. This way, I can reuse the red screens on something else if I need to. The nylon screens are also good for covering up the vent holes I may drill into an IEM shell or headphone cup for tuning (I will place the nylon screen over the hole on the inside to keep dust and dirt out). Many OEMs do this on IEMs and even full size headphones.
