Are REL the most Musical Subs?

All REL are -6dB@ their lowest frequency and were termed  "SUB-BASS SYSTEMS" by the initialed developer manufacturer Richard Edmund Lord on his Operating Instructions.

Regardless of what their product is called today simply matching the REL volume level to your main speakers is not subwoofer low frequency integration. Their dramatic -6dB roll off at their claimed low frequency prevents them from exciting standing wave modes and allows them to be located most anywhere in the room. This locating convenience comes at the cost of discreetly amplifying and adjusting that delicate sub sonic frequency foundation by a few dB that so many recordings poses and you're missing. 

Like most things in this hobby, if you've never heard the potential there's a good chance you'll be satisfied by using a sub-bass speaker simply doubling your main speakers limited low frequency.

"schlepping the sub over the basement testing placement spots,but with advances in DSP I’m now hoping to place the sub next to the rack"

I'm going to assum your potent Paradigm subwoofer was improperly positioned and adjusted resulting in never experiencing the potential of this -3dB subwoofer. Had it been, you would have returned the REL the first day.

With the sub temporarily located at the listening position a low bass cadence is played through both the speakers and the subwoofer. You simply walk around the room making note of the standing wave modes (usually along the side walls) where the low frequency cadence tones sound their best.

DSP is not a panacea to ameliorate the improper but convenient subwoofer location.

Its an amazing thing to watch while the low frequency cadence is being played through the speakers and the listening positioned subwoofer to see the BINGO! on the face of a novice when they hear the standing waves bass mode in THEIR room for the first time.

THIS is the first step towards subwoofer musicality. Sub-Bass Speakers need not apply. They just don't go low...so, no.

I replaced a pair with a single RSL 12S, and I could not be happier. My room seems to embrace the change and works coherently with my speakers, using the hi level connection. To each his/her own.

Let me put a word in for the much more affordable Hsu subs.  In my opinion they are musical and very economically priced.  I use a pair in my video system but have also listened to them in the main audio system.

https://hsuresearch.com/products/vtf-tn1

In a word: "YES" - the REL is the King of the Hill when it comes to integrating a sub into your audio system. I like the Speakon connection to my amp's speaker taps, and I've always found that the REL "sub bass" unit brought a wonderful balance across the frequencies, while also adding some transparency and openness to the overall sound. 

I currently use 6 JL Audio Fathom 110’s, 3 stacked on each side next to my YG Acoustics Hailey 2.2’s in a two channel setup.  No boom, just a natural clean bass that complements the speakers in my living space.