Its not ideal and safe to mount an amp on a sub box. Thats mainly because vibrations from the subwoofer box can cause the internal components of the amplifier to damage over time. However, if your sub box doesnt flex too much or vibrate excessively, you can mount an amp on it.

Is a 100W subwoofer good?

The watt rating is a rough guide to how loud a subwoofer might be able to play, but it doesnt have to be louder. You always control the volume. In fact, a 1,000-watt sub may sound better at regular room volumes than a 100-watt sub. Though quite common, 100-watts really isnt that much when were talking subs.