I went from two REL 510s to a single Carbon Special and in my system it resulted in an upgrade. Perhaps I never had the 510s perfectly dialed in, but how much can one futz with placement and settings before calling it good? The Carbon Special immediately played deeper and faster, and I couldn’t detect any drop in ambience and soundstage by switching to a mono sub.
One thing for sure is the 510s were significantly faster and more defined than were my previous Rythmik L12s despite the latter having servo controlled woofers. This probably shouldn’t be surprising though as a single 510 retails for >$2500 while the L12 is now something like $699. I’m definitely a REL convert now. They prioritize 2-Ch music listening when designing their subs and that comes through in their performance.