Can I make my stereo play mono by...

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.

I find that using a pair of speakers this way for mono is much better than a single speaker.

Only two-channel tube amps can be parallel-connected! This will give double the power for mono use. Never parallel the outputs of a two-channel transistor amp! Some of these amps have internal bridging circuits that will allow conversion to mono safely.

Buy a preamp with a mono function switch, if you don't have one already!

Put the speakers next to each other if your pre doesn't have a mono switch, that will nearly be mono. Alternatively, if you have the freedom to set them up properly and enjoy the stereo, search for " Sumiko master set" this discribes a systematic approach to setting them up in the room and getting it right using nothing more than a CD with a good range of bass notes and your ears.