Wiring both L/R to a mono amp

Assuming unbalanced connections, one normally uses two equal-valued resistors to mix stereo to mono. The value is a compromise between the load on the driving stage and the impedance driving the amplifier. You could try resistors of 2.2 kohm mounted close to the main amplifier input.

If you just twist the wires together, depending on the driving circuit, it may work fine or it may distort like hell when there is a big difference between left and right.
 
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