DAC Stereo RCA out to mono Subwoofer XLR in, how to?

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Joined 2018
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Hello there,

Here is my setup,

Optical out from TV to SMSL SU-8 DAC, DAC connected to main amp with a pair of XLR cables.

There is no preamp, I am using the remote from the DAC to set volume, there are no sources other than the TV.

The SMSL SU-8 also has a pair of left/right RCA outs that I would like to feed to my Sanway D1-800D mono full DSP (i.e. can set XO with it, etc.) subwoofer plate amplifier that only has one XLR input.

How can I go from stereo RCA out to mono XLR in?

Ideas?