Musical watts is not a constant.
The key is not to run your amp into clipping or distortion...distortion kills voice coils because of the DC voltage offset nature of clipping that overheats the coil by pushing it partially out of the gap with no movement, thus eliminating the heatsinking ability of the motor structure.
There is a balance between over and underpowering....sure speakers can be overpowered too...but boy would that be loud.
At any given reasonable listening level with a good quality amp, the 400 watt amp will have way less of a chance to clip than a 100 watt amp.
And it will sound better too.
Moderation is the key..and common sense. if it sounds distorted or scratchy...turn it down.