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.)
128x128hilde45
@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   
Yes you place between your pre and monoblocks, you can set the crossover if you want . You send stereo to the amps through channels 3 left and 4 right. The unit mixes stereo to mono output on channel 1 to the subwoofer. In the future if you get another sub you can send the second to channel 2 for stereo. Is also helps on integrating subs.
@djones51 -- Thanks. That may be helpful.
@jl35 Agreed. I got this $1800 sub used for $900 because it just wasn’t in my budget to spend more. It’s now hard to find. I might sell it and get two lesser subs (maybe smaller REL’s). But for my present room, this single REL is definitely powerful enough, though as you and others have pointed out, it’s not optimal.
the point i was trying to make hilde45,  is that one sub can sound great, and if you don't have hum issues, the mono blocks are not an issue...I have mono blocks and one REL and am very happy with the results...