I have a subwoofer, which takes a single (mono) input. What comes out of the stereo is, of course, a stereo output. I’d asked around for quite a while how to fix this problem, and found out that a simple Y-adapter will not work.
I finally found a discussion where someone answered the question (BTW, some searches are very difficult because the keywords are too common — such as stereo, mono, and speaker). Here is a link to an inexpensive product that will join your stereo subwoofer signal into a mono signal: Stereo to mono adapter