Best to sum both channels, since the bass is still in stereo.
(Well, OK, older vinyl records often blended the deep bass to mono, but you can't count on it - especially with more recent recordings.)
Most sub amps have stereo line-level inputs, and the amp will sum the channels internally. If yours doesn't, look up this thread on the Audio Asylum: https://www.audioasylum.com/forums/tubediy/messages/26/264261.html