substantial increase in tonal density, imaging and dimensionality with a properly integrated sub creating a far more engaging lisening experience.
For me, that was the "magic" as opposed to additional "punch" and "rattling" low frequency output. Perhaps that explains why REL is considered "musical" and often preferred by those folks who are serious two channel hobbyists.
Recently purchased a REL Classic 98, which sounds more similar to my previous Rythmik F12G. Both added the tonal density and dimensionality that you mention, but somewhat lack that additional “punch” that perhaps SVS and similar subs offer. In light of this, I’ve found that you can have the most impressive sub on the market, but if it doesn’t integrate well into you 2-channel system, it becomes too noticeable over time. I think it also depends on how much that mid bass is desired. Cohesiveness through integration, as well as the convenience of REL’s SpeakOn connector helped with my preference of REL over previous subs. Perception is that the REL is more of an extension of my main speakers than a separate speaker.