For the 10 ohm resistors that you substitute for the woofer load, I would think that 40 watts should do the trick. Use four 40 ohm 10W resistors in parallel. Then, when you tap off the woofer output across the 10 ohm resistor to send the signal to the input of the class d amplifier, you will need another dropping resistor to drop the voltage by about a 10 to 1 ratio so that you do not overload and distort the input stage of the class d amp. The value of this resistor will depend on the input impedance of the class d amp. Measure the class d amp’s RCA input, using a DMM, from center pin to ground to see what you get. The class d amp will have an input resistance somewhere between 5k and 50k. Lets say it’s 10K, as an example. So the drop in voltage will be a simple L pad calculation driving into the 10K resistor.
EDIT: After playing music for a couple hours at fairly high levels, put your finger on the resistors to see how hot they are getting. If too hot, you might consider going to a higher wattage rating.