How much power is to much power?

A SET amp may not control a woofer at higher SPLs but that has little or nothing to do with damping, which is a function of an amps low output impedence.
TEST:: take a bare driver...woofer prefered. Thump it with your finger. Rings nicely, right?
Now, take a short piece of wire and short out the connections. Thump it again with your finger. Little or no sound, right?
The woofer basically damps itself when its 'load'...the output of the amp, has low enough impedence. The rest goes up as heat.
As for control, doesn't audible distortion play into this? If you had a dual trace scope connected and could compare the input signal and the actual speaker motions, you could visualize it. the traces match?=good 'control'.
Traces don't match? Bad control.

Exactly Magfan. High power with low distortion equals control. A high DF helps only slightly in the control of the driver. The amplifier starts the drivers, accelerates them and decelerates them and stops them, all at a complex variety of frequencies. Doing all that with low distortion and ample power is key to control of the drivers.

Dave

DF is simply load impedance divided by source impedance.

Nothing more.

I think that DF is slightly more than nothing, but not near so much as most audiophiles attribute to it.

Dave

My understanding is Df matters but there is a wide tolerance in values that will work in any particular application.

You have to listen and judge. At least with dynamic drivers, if the bass sounds mushy or soft, a higher damping factor may help but the actual value does not matter that much.