Mixed-mono mode are possible for both bridgeable and able to drive low impedance loads amps. This will also involves driving a pair of speaker in stereo mode as well as simultaneously driving a single speaker in bridged mono mode off of one pair of the channel of the amp.
In order to do this you can connect a speaker or subwoofer in a normal bridged mode to an amp, and connect the the left and right stereo speakers to the left and right stereo channels.
The amp must use both input channels in bridged mode. Many amp in bridged mode will simply copy and invert either the left or the right channel. This will make a high output to the mono speaker, but it will also eliminates the mixed mono possibility the lose of one channel.
Use passive crossovers when setting the amp in mixed-mono mode to prevent amp overloading. Almost all new amp can be set with mixed-mono mode because when using passive crossovers, the impedance seen by each amp channel is the same across the whole frequency spectrum.
For example, a typical 2-channel amp stable to 2 ohm stereo or 4 ohm mono connected to a sub with a low pass crossover, say at 100Hz, then the amp sees a 2 ohm load on each channels from 100Hz and down. When a full range speaker speakers are connected with a high pass crossover, say at 125Hz, the amp sees a 4ohm load on each channels from 125Hz and up. The amp is protected by the passive crossovers from seeing more than one speaker on either channel at any given frequency. Surely between the two crossover points the amp does see more than one speaker, that is why when using 4ohms speakers the load on the amp dips to 1.33ohms.
