Hi again.

First: If your amplifier has a subwoofer output you don't need a separate lowpass filter for that. That is built into the amplifier. You only need a 2-way crossover for each woofer and tweeter set. Normally the filter in the amplifier will approximately match the natural low-end rolloff of the woofer. It's not true that the volumes are summed. That only occurs for monaural material. In stereo, the summing only occurs for instruments/voices in the center of the stereo field.

Second: Generally, the woofer is the least efficient part of a system so it's not likely you will need to "tame" it with a resistor. Yes, it's fine to parallel resistors to get greater dissipation capability. If you decide to use resistors, you won't need 100 watts. For example, if you put an 8-ohm resistor in series with an 8-ohm speaker, the maximum power for each is now 12.5% of the amplifier's 100-watt maximum output (-6dB). Since music is not continuous, a 10-watt resistor would be more then adequate.

Most amplifiers have a subwoofer level control. Here is an example. You'd use the "bass" control to adjust the subwoofer without using any external resistors.

Hope this helps!