The power delivered to the speakers depends on the power supplied to the amp. Obvious, it may seem, but not always taken into account.
On the output side of an amp - any of them, really, but definitely with class D - is a switch.
In the case of these three, that switch can do two things: connect one side of the speaker to the positive rail of the incoming power, and the other side to the negative, or swap it round.
The power delivered to the speaker, then, is effectively (the voltage of the power supply) squared and divided by the speaker's impedance (and fudged with a factor of two for sinewaves)
So, for more maximum power, raise the supply voltage. For less, reduce it.
The lower power amps you mention can't handle as much voltage as the higher powered one - that's partly why they're cheaper.
But you don't have to give the 20W one its full 14v. If you only let it have the 5v that one of the others wants, it'll sound just as weedy.
If it's then too quiet, you can up the supply - something you couldn't do with the lower powered ones.