I am rethinking my review process for the PRM since it is user tuneable and I want my review to be complete. Now it is just a matter of doing what I need to do ;)
I can list the technical differences between the PRM and IERM, but I can't say for sure how this will change with different tunings, at least at this time, so please take this with a grain of salt. The IERM has a similar presentation to the soundstage space of the PRM, but the IERM midrange is a bit more forward, not quite as spacious, nor as 3D. Vocals on the IERM are a good deal more forward due to the brightness vs. the tuning on my PRM. Both image quite well, but the PRM is cleaner and clearer within the soundstage due to a better focus. Clarity is similar between the two, and the PRM has a bit more instrument detail as well as better recreation of the black space within the presentation. Note attack and decay is more natural with the PRM, especially in the treble where the IERM can be a bit quick on the attack and decay making notes sharp. When you combine the little things the PRM does better than the IERM, the resulting sound is more organic, natural, and realistic.
The above comparison was done with the DX100 (1.1.7) and I will be comparing these two more with different sources and have a better feel once I compare with more of my CIEMs. They may have one driver in common and the single bass driver of the IERM looks about the same size as the dual bass drivers of the PRM.
I am not sure just how much the tonality can be changed as there is a set sound signature, at least in certain ways, but that is part of what I plan on figuring out for my review. Sorry I can't answer the Th900 question at this time (and haven't hear it so I can't answer that directly anyways).