You are correct in your signal path idea, however...
Any external amplifier will only put out a signal at its maximum rating, no matter how big the signal is going into it, so all you'll ever get out of a 100wRMS per channel amplifier is 100wRMS per channel.
What has to happen is that the amplifier sensitivity needs to be adjusted so that the maximum input signal level means that the maximum output is at the right level. There are VERY few amps that don't have this.
If you have this set wrong, you will either be running the amplifier too gently and not getting the best out of it (IE, your 100wRMS per channel amplifier might only be putting out 50wRMS per channel) or if it's adjusted too much the other way, you will overdrive the amplifier, resulting in something known as "clipping" (the waveform will have a flat top and bottom to it) which will eventually destroy speakers, even if they are the correct power rating for the amplifier.
Very important you get this set up correctly.