I think the clicking is changing the ohms setting while playing to power the m5s maybe. When I changed the ohms setting the receiver clicked the same way..
Switching the speaker impedance automatically while the music was playing would not be my first design choice. Your changing the voltage rails on the fly … trading voltage for current. I guess if the caps are big enough the transition would be smooth. If they are doing that (auto detecting it) and your speaker demands high current at some frequencies and needs a higher voltage at others (to avoid clipping) … then you’d get into this situation when you play this speaker louder than the receiver can cleanly produce … best guess
Personally I’d put a hard impedance switch on the back myself. it’s simple.
