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.

@nyev I have a pair of REL T/9x's paired with Harbeth 30.2's and I'm really happy, they just add a little bottom end energy. My comments are set up related only...

- Placement close to each main speaker is desirable. They are acting as extended bass drivers not surround-sound sub-woofers.

-Based on the above, phase is irrelevant. Set to 0.

-The high-level Speakon cable connection is recommended to reduce support calls because EVERY amp can support this method. The way to go is a line level rca stereo sub-out but obviously not all amps have this feature. This was a significant consideration when I recently upgraded my amp, I went with Raven Audio. All you need is a good quality pair of stereo RCA interconnects that come as a separated pair (E.g. Raven's). Don't waste money on overpriced subwoofer cables.

-If you have to use the supplied Speakon cables, trim them. If you still end up with a hum after floating the ground (or not), feel free to private message me for an easy fix.     

Given the OP is not interested in Home Theater, and not looking for earth-shaking bass, the largest may or may not be the best for his specific application and objectives.

According to my local Hi-Fi dealer, he stocks the T/x line because they are designed with a focus on stereo music applications (he doesn't sell Home Theater equipment). The REL online speaker finder and marketing language support this.

Other models are clearly Home Theatre focused. 

when i get a notion to respond to the OP's slightly unusual goals or the entusiastic brand fans you chime in and I chuckle in agreement. 

I so empathize in dealing with the shear stoutly centerd weight of most subwoofers and ones ever diminishing abilities. My subs and Bass amps/cabinets are on wheels. I don't leave the house with anything bigger than my tom drum sized upshot 10" cabinet and cigar box sized switching amp. 

I'm enjoying a notcable differnce in the presentation of my two main system subs which are now on DYI foam topped platforms with large soft foam casters. A stunning reduction of low frequency energy migrating through our homes structure and slightly better definition at the listening position. 

Anyway, ask for help and bend those knees.

This reminds me of a Kamala Harris speech.

+1

@m-db From a San Clemente California resident, you're welcome.

@nyev To satisfy my own curiosity I did what someone here advised and I called REL. I also sent them an email describing my set up and the person who called me back had the email in hand ready to review with me.

IF the goal is augmenting bass in a HiFi stereo set up and NOT earth shattering sub-bass effects for HT (or a nightclub in your home) then GENERALLY then RELs close to the mains, toed-in, phase at 0, crossover set just under the low frequency response of the mains is a great place to start.

Close proximity to the mains is the best chance of integrating timing.

Tweaking crossover and gain to get those settings just right for the room/system does take a bit of time.

He also said if you have a sub-out from the amp/preamp then try high-level and low level connections and use whatever you think sounds the best.

Thanks for the post, I found it interesting and one of the links provided encouraged me to experiment with the 80Hz filter built into my amp.