Pair of Rel S510’s or Carbon Special’s?

Question for OP -- is your goal unequivocally to buy a REL or to create the best bass response possible in your room? Many posters here only mention or advocate for REL, so I may simply duck out of this conversation. 

Rythmiks have adjustable phase which is very helpful for dialing in the subs and you are not paying the premium for a REL. My experience.

"I was able to get results that I could live with from any position I tried; thanks largely to the flexibility offered by the F12SE's broad array of setup options. In my case, "results I could live with" means a smooth transition between mains and subwoofer, bass that doesn't localize to the sub, with standing wave peaks minimized (or eliminated). Getting back to the Rythmik's setup options, there are too many to catalog them all, but here's a few that I found most interesting/useful:

Low Pass Filter: Unlike most subwoofers that have a single knob that selects the crossover point, Rythmik provides a switch that selects a general crossover point as well as a 2nd (12dB/octave) or 4th (24dB/octave) order slope. Meant to be used to determine if the subwoofer will be receiving a full-range or a bass-managed signal, it's also useful for mating with sealed or ported speakers, each of which roll-off at different rates. A crossover knob is also provided for fine tuning.

Delay/Phase: I'm not sure if this is any different from a typical subwoofer phase adjustment, but Brian has a completely different approach to how this should be used. Instead of simply setting it where the output is loudest, he provides a formula on his site for setting it based on the subwoofer's distance from the mains by adding delay. Of course, delay can only be added (subtracting delay would require time travel), so nothing can be done to correct situations where the subwoofer is farther away from the listening position than the mains.

Parametric EQ: Extremely useful, and by far my favorite feature. Parametric EQs are common in high-end subwoofers, and I never want to be without one again. Standing waves will forever be the enemy of smooth, well-integrated bass as long as you are listening indoors (and your audio system isn't in a barn). The EQ made quick work of taming the nastiest of these peaks in every placement the F12SE found itself in."

https://www.enjoythemusic.com/magazine/equipment/0709/rythmik_f12se.htm

@nyev 

I am a bit worried about positioning.

This is why my post about Rythmik is quite pertinent to you. If you are already married to REL, then you can disregard. I have both. They both sound good, but Rythmiks solved the room integration puzzle.

@nyev 

 interesting subs and delayed phase feature, but to me it sounds like in my case I shouldn’t need a delayed phase feature since the subs will be practically in the same spot as my mains.

Mine are, too, but in order to deal with room modes, the adjustment on delay/phase is not identical on the two of them. After I positioned them, optimally, I then discovered that in order to get the best bass response, they need to be set differently; one wants to be at about 120 degrees and the other is at about 165 degrees. A REL would only allow 0 or 180. 

@baylinor ​@rhg3 ​@onhwy61 ​@nyev 

The phase setting came out real handy when I realized having both subs by the mains did not work....I moved the left one down a third of the length of the left wall and with the phasing dialed in for this one, I no longer had any spot in the room with overly booming bass. 

I understand brand loyalty so if you like REL then who am I to say otherwise?

all the little details...are all secondary to proper subwoofer positioning.

To add a bit about my experiences. 
I have both REL and Rythmik. I had REL alone for a long time and moved it everywhere. I even tried putting it 4 feet in the air (tricky) and measured that position in various places. In the end, the REL sounded good but various problems were ineliminable. 

I added two Rythmik subs and started moving those all over the place. I had a backache every day for weeks. Seriously. Positioning was not enough. Something more was needed.

The breakthrough was placement of the Rythmik subs a bit behind my mains AND playing with the various levels (gain and phase) until the bass from 20-300 Hz was within 5 db of the average. The REL is connected with Speakon but it's out near the listening position with the crossover set much higher.

Bass bloat went away, things got full and tight, and the subs disappeared.

People with REL's have good experience and report them; often, they overestimate how universally applicable REL's might be to a particular situation because of brand loyalty. That does a disservice to the OP, here, because what he will have to do is buy multiple (more expensive) REL's to dial in his room. (Which might not even work, and then he's looking for bass traps.) Only someone with a dream of being a REL fanboy would fork over that much dough; if a sonic goal is really what is sought, one gets over brand loyalty. After all, if one had a boat they needed to tow, would they buy the brand they liked or the one which could actually tow the boat?

@m-db  @thyname 

Thanks -- I can see that the main goal is ultimately to buy a REL brand product. I was confused because the OP mentioned "speed and bass definition and quality...positioning constraints...and precision, ambience, clarity and space" Thus, I thought achieving those acoustic goals might supersede buying REL, but I guess not.  

@nyev REL makes fine products; they're also quite popular. I love my REL. But the issue for you is the constraint placed by positioning and whether REL's -- for their price and lack of flexibility with settings -- can get you where you want to go. You're hearing from those who love their REL and some (like me) who love their REL but don't think that a REL is necessarily capable of working in every situation.

OP, the subwoofers produce waves that bounce around the room. The simple picture of a sub placed next to speakers doesn't do the complex physics justice. Take a look at this thread from 2021 -- there are many good posts on it.