I'm pretty sure you can just connect the left and right channels so instead on L/R and ground you'll just have the single channel (combined L/R) and ground. I have done that before when I connected the stereo output of an iPod to a mono amplifier and then to a single speaker. It seemed to work fine for me.
The end quality was not as good as just plugging in stereo headphones but it was hard to tell if the loss in quality was due to the method I described above or to my crappy amplifier design. At any rate, the loss in sound quality wasn't drastic. It was slightly noticeable to me but I'm no audiophile.
Don't short together the stereo outputs unless you know their amplifiers will not blow up.
A couple of resistors make a good mixer for stereo to mono if you have a power amplifier to feed and if it can drive your speakers.
You should not reduce the sound quality but mono sounds different than stereo.
Then why are bridging the left channel to the right channel? The correct way is to use three amplifiers, a high-pass for Right, a high-pass for Left, and make the third a summing amplifier with a low-pass response to drive the center (bass) speaker.