How to connect monoblocks to a single REL sub with ONE RCA input


I just purchased two Quicksilver 60w Monoblock amplifiers and a LineStage Preamp by QS.
To connect this REL 328, normally one uses connect their Speakon cable (Red, Yellow, Black ends that connect to Right+, Left+, and Ground) or a SINGLE "low-level" RCA in. But this preamp doesn’t have a single sub out.

REL’s advice is to buy a second sub and attach one sub to each monoblock. Well, maybe I’ll do that, but in the meantime, I’d like to be able to hook up what I have.

Should I orient the amps so that the speaker terminals are close enough together to attach the strands of the Speakon cables? I have been warned by REL not to hook up high level to monoblocks because it will create a ground problem.

An alternative is that I get a Y cable that will bring two RCA outputs from my Preamp into the single RCA input on the REL?

Or is there another alternative?

Advice appreciated. (But please try not to tell me to just buy a second sub.)
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Regarding your recent question about the device suggested by @imhififan , when I looked at the links he provided what particularly gave me a fair amount of confidence was the statement on the packaging that can be seen in one of the photos, that "it provides mono output while retaining L/R input isolation." I interpreted that to mean that the left input and the right input are isolated from each other.
Thanks @almarg for clarify, the MCM converter is utilizes an independent 1:1 ratio audio transformer for EACH channel, so the left and right inputs are isolated from each other.
@almarg Great to hear. I certainly want to avoid any issues. I *assume* that if I start to hear a hum that I’ll just disconnect the setup. I asked QS in several emails about grounding to preamp chassis and that suggestion wasn’t even taken up in the (very sparse) replies.

@daveyf
Is that set up the one suggested by REL?

Not for monoblocks. If you look back at some of the REL links people shared above, you’ll see that REL suggests NOT connecting the sub's high level connector to monoblocks. They recommend using the "low-level" input. My problem (as described above) is that my preamp output is stereo and the REL I own only has a single RCA input.

 My sense is that the advice they’re giving here is somewhat inexpert and/or poorly researched. I am not confident that my question got to someone with sufficient knowledge and/or they didn’t really stop and think hard about the question. Their suggestion of a Y cable would (as QS, almarg, told me) collapse the stereo field for the main speakers. While I sympathize with REL’s predicament of having many customers with a wide variety of products, it would be better to say "we’re working on your question" than to give erroneous advice.

I used that exact set up on my prior ARC stereo tube amp, and it worked fine. I also use that basic set up on my ss balanced stereo amp.

I’m assuming by "same set up" you’re saying that you used the high level REL connector. The issues raised by my situation are critically different, insofar as they pertain to grounding the subwoofer’s connection on two separate monoblocks.

@djones51 -- Interesting. Just reading the manual. "3.8 MONO/STEREO MODE The 2 Way Advanced 21 plugin supports mixing of two output channels, allowing this plugin to be used in “2.1” applications where a mono subwoofer signal is derived from a stereo signal. To access this selection, click on the System Settings tab."
REL always recommends 2 subs for sonic reasons (not just more          bass). The only reason they state on their website for not using 1 sub with mono blocks is the greater chance of hum