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

I say, buy the most expensive REL you can afford. Of course, bass quantity is important, but IMO the most important quality of a music-focused sub (like REL) is speed. The drivers have to be very quick and must start and stop on a dime.

Based on my listening, I think Carbon Specials will give you more speed. And I say this as someone who owns a pair of S/510s which are no slouch either :)

Good to know, thanks. Have you heard the Carbon Specials in comparison with your S510’s? Based on their specs I’m sure the Carbon Specials can go deeper and louder, with greater speed, but in my case I only plan to have the subs add very minimal effect. I’m not actually looking for deep full bass you can feel. Just a bit of lower end QUALITY support is all I’m after. But maybe the added speed and detail of the Carbon Specials still matters in my application? Or maybe the difference would not be noticeable?

I should say up front, 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!

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