FYI: The 20k has twice the RMS power of the 14k. The 14k has 1 PSU, the 20k has 2 (one for ch 1-2, and another for ch 3-4).
That's why the 20k requires a 60a breaker, and the 14k only requires a 30a breaker.
If it is a dedicated breaker you could do 240V 20A. You just need a 240V wallsocket and breaker, and 240V custom plug on the clone. (<$100 at walmart or any hardware store...)
[and buy the 240V clone of course!]
120V is 1 hot (black) +120V, a ground (copper/green) 0V, and a neutral (white) 0V.
240V is 1 hot (black) +120V, a ground (copper/green) 0V, and a 2nd hot (white or red) -120V.
At least in USA and Canada it is...
"Technically" only a certified electrician should be doing the install.
Just make sure you have the mains-breaker turned off before working on the inside of the service panel.
For the same AWG of wire: 240V will provide 2x the watts.
All romex is rated for like 400-600v, so you're all-good there.
The breaker size MUST be the same value, you can't increase it.
The breaker protects the wire from melting, and thus indirectly protects the house, it doesn't protect the appliance nor the humans. Wires are current-limited, not voltage-limited.
The 240V wallsocket ensures Darwin's don't plug 120V appliances into them, for obvious fire/safety reasons.
(To be fair: Generally... you can't see/taste/smell radiation, carbon monoxide nor electricity.