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

@nyev My pleasure. Good news on your RCA sub-outs. The high-level input driven from the speaker binding posts passes through a resistor that essentially turns it into a low-level input anyway. Note that gain control on the REL is the same for both high-level (Speakon) and low level (RCA) inputs. (There is a dedicated RCA input with it’s own gain control but that is only for low-level .1 surround sound). There are zero issues with using the RCA low-level inputs unless your RELs are really far from the amp.

I a/b tested high and low level when I got the new amp and there is no question that in my system the RCA/low level sounded better.

Miller Carbon would buy 6 more subs.

This has been a very helpful exchange.  I have resisted subs for for more than 40 years (since they started?) and am still doing so, but every now and then I am tempted in to have another look, as here.

@nyev , all the little details about power cords, which sub model, high vs. low level connections, phase control, etc. are all secondary to proper subwoofer positioning.  If your subwoofer position is wrong, then the other stuff won't even matter.

A simple test would involve moving your current loudspeakers to the proposed subwoofer positions and run a series of bass tones to evaluate.

@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?

I would go with the largest REL's you can afford.  You can always dial them back.  I still thin the high level connection designed by REL is incredible.