A stereo amplifier that is run in "mono" mode will essentially double the pull on the power supply. You are, essentially, running this stereo amplifier in bridged mode. This means that each channel of the amplifier is only seeing half of the speaker impedance. So, if you are running a flat 8 ohm speaker, each channel would be seeing a load of 4 ohms. That is why the power seems to double. At this point, you will have to be very careful on what speaker you choose to drive. Many speakers that are rated at 8 ohms will actually have an impedance curve that may drop down to as low as 3 ohms in certain bass / midbass frequencies. This means the each channel of the A21 running in bridged/mono mode will see only 1.5 ohms. This can put undue stress on the power supply and may cause heat/overload scenarios. The sound quality here may suffer a little as the power supply may not be as beefy as you need. Also, you may force the voltage rails that power the output circuit to running higher than they really are designed for. This specific fact is why is it never wise to run an amplifier in "bridge/mono" mode unless it was specifically designed for that.
It’s much better to run a true monoblock where the power supply rails and output circuits were designed to see a specifically lower impedance on the speaker output section.