I see that measurements are being shared by/from one of the UK retailers, so I guess I can share another comparison using black atom modules.
Yes, RN6 is a "leaner" lower-mids version of XE6, but that change in tuning makes a world of difference if you want a tighter punchier bass without loosing the power of the original impact and sub-bass extension, and if you want more clarity in mids/vocals where RN6 excels over XE6. By tuning lower mids to be more neutral in RN6, you hear more contrast between the bass and the mids, giving the bass more control and better articulation and even having the same perception of impact as XE6. It does complement XE6 tuning which has bass that sounds fuller and more relaxed due to a longer decay of notes. One of the differences I found between XE6 uni and ciem, relative to my ear anatomy, was ciem having more clarity in mids due to a better fit. XE6 uni has a bit more heft and under its weight and considering open grill of kinetic DD driver, the seal was looser. XE6 ciem improved that, but RN6 takes it to the next level due to updated tuning and lighter shells (which result in better fit, for my ears). Btw, RN6 and XE6 shells are the same size.
With black atom modules, RN6 has even a bit deeper sub-bass rumble than XE6. And, I love red modules with it as well, also recommend getting it with RN6. With red module you get even more texture in sub-bass and upper frequencies sounding more open. All subjective opinion, but to my ears RN6 hit the sweet spot!