As I said before, I would recommend using 2-3 times more power for bass than other speakers.
Say you get an amplifier that does 4x100 at 2 ohms and 4x50 at 4 ohms. If you have 4-ohm front and rear speakers, then your total power would be 4x50 = 200 watts.
Based on that, I would suggest going with 400 - 600 watts for bass. This is just a rule of thumb, there are many factors that play into designing a system, but this is a good starting point for a well balanced system.