REL Subwoofer: Speakon or RCA?

While while I am not completely sure, I am under the impression that the Speakon cables are the way to go to get maximum results. 

Speakon sounds significantly better.

Everything has a price for achieving a given effect. YMMV. 

I’ve had Rels for many years. Speakon sounds best to me for music.

I'm a REL dealer. Speakon is way better because the amp in the REL will emulate the character of the amp so the blending and seamlessness is much better.  It will sound like the amp in the REL is a Rogue.

I've  used Speakon stuff for years in pro audio so when I acquired a couple of RELs over the years I adopted their philosophy of the high level input for the reasons mentioned in audiobunker's post, and because I like Speakon Neutrik plugs...I make cables for the RELs from Canare Star Quad, AQ spades, and Speakon plugs...shrink tubes galore. I have to "stack" the spades on the posts of my amp, but that works fine as the main speaker's cables are bananas. Neutrik...made in Lichtenstein...

Here are a few videos REL Technical support sent me when I asked basically the same question for my REL Stadium III
https://rel.net/before-connecting-placement/ Connecting 
https://rel.net/determine-whether-to-use-high-level-low-level-and-lfe-1/ Which connection 

https://rel.net/how-to-connect-your-rel/ How to connect 

https://rel.net/how-to-tune-a-rel/ How to tune 

Speakon is the way to go...BUT be very careful how you do the set up and look at the videos above. Speakon is a balanced connection and that needs to be considered when hooking up to a balanced amp. 
There are other variables depending on your amp type, but the videos do a good job of covering them.

Long runs of RCA single ended cables aren't a good idea, but the 100,000 ohm REL "signal stealing" High Level input system doesn't seem to care how long the amp driven cable is. Plus it's easy to build them yourself.

Using Speakon is the only good thing about REL. If you were to use RCA, there’s a lot of other subs that perform much better.

@mzkmxcv

great post, you know this as a total fact and you have heard all of the possible combinations of Rel and ancillary gear.
Nice to know your opinion...even though you conveniently forgot to add....imho.
IMHO.

@daveyf  
  
Not an opinion, based on pure performance, an equally priced sub from say Rythmik, SVS, PSA, JTR, etc. will easily outperform it. REL doesn’t offer deeper extension, louder output, better microdynamics, more musicality, etc. In today’s age, you can get better for your money, they may sound good, but not as good as it’s modern competitors.

REL SUB-BASS SYSTEMS are not designed for use in applications where the main speakers are externally crossed-over to eliminate lower frequencies from the mains, to substitute for poor bass performance of main speakers, nor to induce terrified responses to explosions while viewing the latest mindless action movie. In those applications, mzkmxcv is correct that REGULAR SUBWOOFERS provide better results than REL.

REL SUB-BASS SYSTEMS are designed to provide room pressurization of frequencies around and below 20hz and to slightly augment bass performance of full-range loudspeakers in that region. The net effect of a properly implemented REL is audibly improved soundstage and a solid bass foundation for music with near perfect integration with most full-range loudspeakers when used with the hi-level connections from the amplifier’s speaker outputs. If that is the intended application, nothing else compares to the RELs IME.

Dave

Over the years I've heard systems with various subs from old Velodynes to Vandersteens and others not having a V in their names, and RELs certainly hold their own compared to anything out there, including my 92 lb monster studio sub that's actually flat to 19hz...RELs are great designs, period. My 2 RELs cost 200 bucks each used and in mint condition by the way (Q150e I bought first maybe 5 or 6 years ago, and Q108 MK II I found maybe 2 years ago), perform perfectly and are musically transparent in the sense that when properly set up they simply disappear. 

@dlcockrum  
  
Nice breakdown.  
  
One thing I would like to touch on: 
  

REL SUB-BASS SYSTEMS are designed to provide room pressurization of frequencies around and below 20hz and to slightly augment bass performance of full-range loudspeakers in that region.

You would have to go really high up in their price range to get that low. The $1300 T9i is only be down to 28Hz at -6dB. And while we would are disregarding the benefit of the REL’s integration, the ~$500 Rythmik L12 (which is very musical for the price) is rated down to 12Hz at -6dB, for all those pipe organ lovers, something even the $7500 REL reference model doesn’t quite achieve.  
  
But it is true most music doesn’t go below ~30Hz, though it’s still nice to have a subwoofer that can still handle that without strain, as those specs are given at low wattage, and they very much so would have some compression on their low end when playing back at reference levels.