Typically with multi-BAs the main difference between IEM and CIEM will be due to a fit with a variation in insertion depth and the seal: CIEM will have drivers closer to your eardrums and a better seal, versus IEM sticking out further with a seal controlled by eartip selection. Here, TIA driver is placed in the nozzle and not tubed, which turns U/A18 single-bore into a sound chamber. In theory, different material will affect how the sound resonates inside of that chamber, but it's such a short path due to driver being very close to the tip of the nozzle that I don't think metal vs acrylic will matter. Again, I'm making an assumption in here, and only have experience with universal U18. Maybe @Barra or others who tried both can chime in with their experience?