Can I make my stereo play mono by...

Never allow 2 amplifier channels to touch via a conductor.

You may have 1 amplifier channel serially connected to any number of speakers:

Amp (+) ---> Speaker 1 (+) (-) ------ > Speaker 2 (+) and so on, until ----> Amp (-)
Do not short speaker + and -

You may connect speakers in parallel, but this has the usual impedance limits.

Of course, this is not really mono. This is 1 channel (L or R) outputting on mulitiple speakers.

The safe way to create a mono signal, if you don't have it in a switch is to use the RCA/XLR cables and sum them there. It's relatively safe only because at 1-2 V you are unlikely to create high enough currents to damage anything.

Technically, the ideal way to do this is via a high impedance summing circuit.

Amplifiers are high current, and they have an idea of what the signal should be.

If 2 amplifier channels are summed by a cable, they will usually fight each other, providing their maximum output current to win.  Imagine there's a trumpet on the left. but not on the right. The two amplifiers will move all their current to change the voltage to what they think is the correct signal. One will fight for the trumpet, the other will fight for silence.

That is why you can't just use a wire to connect the outputs.

... thank you for your explainations - in this case I guess Electrical Engineering trumps common sense...

Leave your pair of speakers in their normal location. Use the "mono" function on your preamp (if it has one). You will have a nicely centered mono image between the pair of speakers! Leave the amp connected as usual for stereo.