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

I have 3 s510s in my room....I'd go with the 2 carbons if you can swing it

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

Take a look at Perlisten subwoofers.

D212s (dual, push pull, 12" carbon fiber woofers), 3.0 kW amp (retail $7,000), or

R212s (dual, push pull, 12" glass fiber woofers), 1.3 kW amp (retail $5,000)

There is also the D15s (single carbon fiber 15" woofer, 2.0 kW amp). Retails for $5,000

Subs will be positioned about 6” on the outside of each main speaker. This is the ONLY place they could go…

The chances of this being the optimal placing for the subs is small.

probably all I need is to activate the EQ in Roon and boost the lower frequencies a couple of decibels.. But due to my irrational audiophile sensibilities I can’t even bring myself to try turning it on! Don’t want any app messing with the purity of the source material!

Are you serious?  Think how many times the digital signal has been manipulated before it even gets to your streamer!