Speaking of a sub as "musical" in and by itself is somewhat misleading, and yet it’s not entirely without merit in a system context. Before getting to that however it’s primarily about the implementation of subs, as proper integration with both acoustics and the mains as an outset is paramount.
With that "out of the way" - be that either via a DBA-approach, the involvement of Digital Room Correction, dual subs, symmetrical to mains-placement or asymmetrical, high-passing the mains or not, etc. - I’d say the differences between sub brands as mostly sealed designs is the far lesser factor here, with the varying aspect, apart from the all-important implementation/integration, being more about total capacity (i.e.: displacement/cone area), extension and design. More displacement means less cone movement = lower distortion, whereas more extension from a similar sized sealed package means (even) lower efficiency, which in turn typically implies higher moving mass and the need for more power and power handling. Design choice is important as well as some of them will offer much higher efficiency, albeit at the cost of larger size (Hofmann’s Iron Law).
The aspect about lower extension from similar sized sealed package is not without sonic consequences, I find; those low eff. (i.e.: below 85dB’s) high moving mass, very high power handling and high excursion woofers, even with huge magnets and a ton of power, have a tendency not to mesh that well with the main speakers sounding often too "solid" somehow and with a notable overhang. That is to say: they can sound disjointed and call attention to themselves, less so when being properly implemented overall.
Looking at REL subs they usually don’t extend that super low, certainly not into infrasonic territory, and that tells me they’re using relatively low moving mass woofers which, all things being more or less equal implementation-wise, tend to blend better with the mains - certainly in the more limited context of sealed subs. So, more "musical" with main speakers augmented by subs is aided by proper implementation and integration with the mains/acoustics, in addition to being considerate about what you’re trying to squeeze out of a smaller sized typically sealed sub package.
If you’re really into making a musical sounding combo of mains + subs, ultimately, treat it as a single speaker system per channel and go with a fully outboard active approach (i.e.: mains + subs), high-pass the mains, use a separate quality DSP unit, use identical quality amps top to bottom (potentially power differentiated, but of similar topology/design) and, finally, let physics have their say with the subs themselves; if you want infrasonic territory reproduction, then go the distance and be prepared for prodigious displacement and overall size + a boatload of power. If you can’t accommodate this, forget about infrasonics.
In any case use 2 subs (or more), preferably (to my mind) placed symmetrically to the mains, and use subs with the biggest diameter woofers (or, overall effective air radiation area with horn-loaded variants) you can afford/will. Anything below 12" with direct radiating designs in pairs or more simply won’t do, and the higher efficiency the better.
It’s about physics, design, proper construction and implementation/integration, folks, not the damn brands. For this context and the wishes of the OP nonetheless I’d open up the field of brands and consider them in the light of the above.
