FYI, head units only put out about 12-15 watts per channel of rms/continuous power.
The ratings advertised on the box and in the ads is maximum power output. It's a useless rating, which is a split second of power output full of distortion. It's basically marketing glitz.
You need 10X the amount of power to double the volume, so the difference in actual output between something like 10 and 20 watts is very minimal.
Certain 6x9's will fit by enlarging the hole and/or drilling some new screw holes for mounting. Just watch the mounting depth. It's only 2.75 inches, pretty shallow.
For all the work, you'd be better off investing in a set of components which will produce far superior sound to just about any 6x9.. See this
Link.