HU's are like ~5 watts a channel. And that 50x4 watts you see on aftermarket is going to be a max rating as well. Now way they are fitting a 200 watt amp inside the tight confines of a headunit.
If you want to add some bass easily, just get a sub and amp package and splice wires from the rear speakers into the high level inputs on the amp after you do the power, ground and remote wires, and you have some bass.
There are a lot more ways to do it.
A full new system.
A set (or two) of amped components in the front. Rear fill pulls the soundstage backwards. It can hurt you sound more than help with a sub and amp still.
It all depends on how much you like music and what you want to spend.