Are REL the most Musical Subs?

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.

The dichotomy pointed to with regard to music vs. home theater oriented/labeled subs isn't one to follow with necessity, but rather one that rests with either camp and their potentially self-inflicted limitations. Some if not many of the so-called music oriented subs simply lack the capacity first and foremost to properly handle the demanding low frequency material found in a range of movies on Blu-ray/4K UHD's, while also being restricted into the infrasonic territory. Conversely some Home Theater co-labeled subs may be hellbent on squeezing that last ounce of extension out of a small size factor (albeit typically with larger driver diameter), with all that entails and which may lead them to integrate more poorly with main speakers.

It seems to me though that when overall sub size and driver diameter exceeds that of the more "hifi"-oriented or -accepted sub variants/brands they're automatically relegated into the Home Theater segment (and the associations that follow), which is really a load of B.S. Look at REL: they're small and there seems to be a consensus about their integrating very well with main speakers, hence their popularity in audiophilia. Take then, say, JTR and one of their single or dual 18"-loaded subs of more prodigious size, and mostly there wouldn't be a bloody f*ck of an audiophile who'd give them any serious notice because they're (much) larger, not least the ported variants, and oftentimes more functional looking as well, so naturally they're just about blowing off the roof of one's house, right? Wrong. 

What many still don't seem to understand is that with subs you can "have your cake and eat it too," it's just a matter of proper capacity and sufficient* extension from a design that lets size have its say. A well designed and constructed, large and efficient sub will, as always with some groundwork, be able to integrate smoothly with main speakers and accommodate music and movies alike - in both cases even more so than some puny cube of an inefficient sub (unless with a multitude of them). It's not about either/or (i.e.: music or movies), but doing it right from the outset and thereby covering both bases at once. 

* A word on "sufficient" extension. This is an area of debate, but first of all infrasonics do make a difference with movies, although for proper effect and those very low frequencies to be truly felt you need prodigious displacement and a lot of power. Some also swear by infrasonic capabilities with music reproduction and its deemed importance here, but subs tuned that low to my ears don't always sound as "alive" in the midbass region. I've found that covering down to an honest 20-25Hz area is plenty deep for most occasions, even with movies, without negative impact on the midbass area, and this way one can also maintain high efficiency without ludicrously sized sub enclosures (i.e.: 20-25 cf. per cab). 

@bdp24 wrote:

The Rythmik servo-feedback system also compensates for the increase in voice coil temperature, which all woofers suffer from, regardless of their excursion capabilities.

How would it do so - other than acting as a limiter?

This debate primarily has your usual, lower efficiency and sealed sub design in mind that is built around a small, restricted size factor. As such heat dissipation from voice coils will be an issue eventually, but thermal limitations are not inherent to a wider design range of low frequency augmentation options that have higher efficiency as a core trait (of course with added overall size as a necessary implication) - certainly not in the context of domestic use.

The Rythmik dipole circuit progressively boosts the signal as frequency drops, resulting in flat response to 20Hz.

But this comes at the cost of eating away of the headroom that might (or might not) have been available initially, only exacerbating the issues that come from lower efficiency and what follows here both thermally and mechanically. If you multiply the number of such subs used to appropriately accommodate the clean SPL envelope that’s required, you would be able to at least partially alleviate this issue while also opting for a DBA approach, albeit at a higher cost.

On the other hand, when you have a pair of corner mounted, pro B&C 15"-loaded 20cf. per cab tapped horn subs with 97dB sensitivity (+ boundary gain) that deliver air-shaking +105dB levels at the listening position down to 20-25Hz, all the while exciting the woofer cones to only vibrate a few mm’s, you know you have actual and usable headroom in abundance and a wholly effortless reproduction at any desirable SPL. There’s no way to cheat around that other than blunt, core physics and letting size have its say, and don’t tell me audiophiles don’t need that kind of effortless bass delivery; if they heard it they’d most likely crave it, if it wasn’t for the size of such things.

I’ll say it again: The only people who don’t know how good the Rythmik Audio/GR Research OB/Dipole Subwoofer sounds are those who have not heard it.

The same could be said of those who haven’t heard properly designed and constructed high efficiency DIY sub designs (i.e.: not restricted to one or a few brands), sans all of your EQ-boosting, servo feedback, dynamic limiting, ultra high power demand and other electronic-digital trickery that will not be needed here (other than a quality DSP that will act as an elaborate crossover device). If anything it’s the purist subwoofer approach, and it could be even more so with the use of outboard quality amps instead of cheap plate dittos, etc.

@travbrow wrote:

I thought the OP was interested in a good option that’s available to buy, not a DIY approach thats out of reach to most everyone? It’s interesting to read about the possibilities with DIY though, but to imply any other way is lacking enough not to be worth it is wrong IME.

The thread title and OP’s "... but was wondering what the current thinking is with subs" to some degree - and for good reason - opened a can of worms with regard to whether or how musicality applies to subs. You’re right, 20cf. behemoths are likely not what the OP has in mind to pull off LF-duties in his home, but from my chair what aids overall realism/a sense of being emotionally involved (and thus "musicality") in sound reproduction is exactly a more all-out approach with DIY subs of higher sensitivity, incl. considerations on (outboard) amp choice, DSP, etc.

While a brand like REL is often lauded for making subs that integrate smoothly with existing main speaker setups, my focus is to point at mechanisms behind this that aren’t exclusive to or even accommodated by popular sub brands because of their smaller size and other, and so saying there are in effect more successful ways to have subs aid overall realism and musicality. I’m not implying the OP can’t be happy with a REL or similar solution, however... sometimes what you’re asking can produce answers that seemingly go off-topic, but are nonetheless in accordance with what’s being brought up.

@bdp24 wrote:

On the Rythmik Audio website designer Brian Ding provides exhaustive technical information on his subs. I believe details on the servo-feedback compensation for voice coil temperature variation and other functions may be found in the site’s Technology section. Mr. Ding is not your average hi-fi designer, having a Ph.D. in electrical engineering.

I’m sure Mr. Ding is very capable in his field, but I’m calling out the relevance of servo-feedback in a different context of subs. People seem to believe small subs are everything - and yes, subs from Rythmik, REL or other are small in light of the frequency range they’re called to reproduce - and thus that the likes of servo-feedback is a qualitative measure in every regard, when it isn’t.