A Peavey ValveKing 20MH amp has a speaker output impedance switch which allows selection of either 16Ω or 8Ω speaker/cabinet impedances with 8Ω max on the 20MH.
If you are considering wiring more than two speakers to the amplifier, you need to concern yourself with your amplifier’s ability to handle low impedance loads. If you ignore the way you wire them together you can damage the amplifier
Adding speakers in series increases the overall resistance of the circuit.
Zt = 8Ω + 8Ω
Zt = 16Ω
Use 16Ω setting
Adding Speakers in parallel decreases the overall resistance of the circuit.
Zt = 8Ω
Use 8Ω setting
As long as you're careful with output impedance you can run an 20 watt amp though any drivers greater than 20 watt. This will alows you to extend your clean headroom a bit, but any overdrive will be strictly from the natural tube amp breakup, your attack or your pedals.