Here are excerpts from impressions I've posted a while back on both this thread and Nic's Ranking the Stars thread:
"I had the chance to A/B the XR and the XRA for the very first time, and the differences between them were larger than I had anticipated. I felt the XR had more resolution, much sharper and more precise imaging, a blacker background with greater dynamism (due to the better isolation, I presume), better linearity and coherence, and an overall more intimate and focused sound. The XRA, on the other hand, felt more stretched out, resulting in a better feeling of immensity. However, this resulted in the XRA being less engaging, as the music is more out-of-head, rather than you being in it. The XRA also sounded less resolute, and I felt the masterful balance in tuning that the XR had, was somewhat tampered with, resulting in a less even and less realistic/natural sound. I'd say, the XRA is great if you'd rather sit back and let the music surround you, at the expense of utmost detail retrieval and tonality. On the other hand, the XR is fantastic if you want to actively enjoy/critically listen to your music with ultra-high levels of engagement, clarity, and superior imaging and transparency."
"The XR and the XRA have different signatures and different presentations. The XR is richer, more bodied, and more natural sounding than the XRA. The XRA has the impression of being more airy due to a sparklier treble and thinner lower midrange, even though the XR is pretty airy already. I find the custom XR to lack a tiny bit of air just in the treble; everywhere else it has air in spades. On the bright side, this allows the XR to be more forgiving with harsher recordings, while they get unapologetically destroyed by the XRA.
Presentation-wise, the XR is more focused with fantastic clarity, resolution, and pinpoint imaging. The XRA, on the other hand, has a wider and more euphonic presentation, with imaging that's less precise, because the instruments sound somewhat stretched all around you. Resolution is also not as prevalent because of a less black background, some incoherence in the lower midrange, and the aforementioned phenomena of having details stretched around you. A friend of mine who has the XRA claims the carrot module cannot fully convert an XRA into an XR, even if it can get close."