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

I was in the same position as you, owning one REL sub and utilizing a mono block tube amp. I realized that the value of having mono blocks is to completely separate the signal from the left and the right speaker, with no possible signal corruption as is possible with a stereo amp. As such, it made perfect sense to me to be able to separate the left and right signals that the sub sees into a separate left and right channel for the subwoofer! That is why i think REL suggest this set up ( dual subs with mono blocks) in the first place and it certainly works a lot better this way. I have a stereo amp ( balanced) and with that I use just one of the REL subs, it works fine, but note bass definition is definitely less in this set-up. 

Update from me: I tried the high level. Connected to both mono blocks. No hum. Works fine. (Will it last?)

@hilde45, re your latest update, what is the black wire from the sub connected to?

Also, re the suggestion from QS of connecting the black wire to the ground of the right channel amp, that is a reasonable suggestion. And since I’m pretty certain that your amp does not have outputs which are balanced or bridged, a suitable ground point would most likely be the negative output terminal of the amp. (The black/ground wire of a sub should not be connected to the negative output terminal of a balanced or bridged amp, since in those cases the negative output terminal provides a full amplitude signal rather than ground. Also, there are a few tube amp designs, such as some of those made by Audio Research, in which circuit ground is connected to the 4 ohm terminal, rather than to the negative/"common" terminal). Whether doing that would provide results that are better than, worse than, or about the same as would occur if you connected the black wire to a ground point or chassis of the preamp figures to be equipment dependent and hard to predict. My guess is that more often than not the results would be similar.

Re the mention by @jl35 of the possibility of not connecting the black wire at all, some other members here have reported doing that with fine results. And in fact I recall at least one such member indicating that REL had suggested trying exactly that. The reason that can work in some and perhaps many systems, considering that for an electric current to exist a "complete circuit" must be present from source to load and back, is that the AC safety ground wiring forms the return path in that situation. Again, though, how well that would work is equipment dependent, especially on how or if circuit ground and chassis ground (which in turn is connected to AC safety ground) are interconnected within the specific equipment.

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.

Finally, regarding your question about the possibility of damage, in this particular case I don’t envision any such possibility for any of the approaches that have been referred to.

Best regards,
-- Al

@almarg 
Connections, Subwoofer speakon connectors to monoblocks:

Red to Right +
Black to Right -
Yellow to Left +

Regarding outputs that are "balanced or bridged" I don't know how to tell — but if it's indicative, there are only RCA connectors on the preamp's outs and they run to RCA inputs on the amps. 

Interesting point about the un-connected approach to the black ground wire. If I heard a hum, I suppose I could try disconnecting that wire.

I appreciate your taking a look at the mono/stereo device mentioned by imhififan. In some ways, it still seems like the safest way to go, but I can testify that I tried both high level and low level inputs tonight, and high level is far superior and it's drawing on exactly the signal being sent to the speakers.

I continue to admire the clarity and concision of your replies, and the time you take to research someone else's (my!) question. Thank you.

@hilde45  Is that set up the one suggested by REL? 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...although I place the Black to a grounding connection on the amp...as I don't think the minus is a true ground. Works well too, but this is with just one sub connected, not two.