Use a y adapter from the CDP to the mono amp. This will mix the left and right stereo to mono!
https://www.amazon.com/KabelDirekt-Analogue-Shielded-supports-Subwoofers/dp/B00DI89D3Q/ref=sr_1_2?dc...
https://www.amazon.com/KabelDirekt-Analogue-Shielded-supports-Subwoofers/dp/B00DI89D3Q/ref=sr_1_2?dc...