How can I determine how much secondary output (volts and current) is needed from my transformer and the size of heatsinks I need for a particular desired output power? In my case, I'd like to get 100 to 125 watts out (it can go up to 150, correct?).

Also, the speakers I'm planning to build will, most likely, dip down to 4 ohms for part of their response. Do I need to get the additional modules?

Thanks,
Paul Ebert