Purchase a quantity of 14 of a part number 004-30
from Parts Express. You will end up using 7 per channel, all wired in parallel.
This means that all seven resistors will have all their leads tied together on one side and tied together on the other. In effect, the signal comes into one common point on side A ( we'll call this positive ), can choose a path through any of the resistors, and then exits on side B ( negative ). In effect, the signal is passing through the resistors like it would a speaker. You can connect the resistors aka "dummy load" to the amp using some old zip cord speaker cable or whatever you have laying around. If you like, you can use your existing speaker cables if you can find a way to connect the resistors to them in a secure manner.
Using this specific approach will provide you with 70 watts of power handling and a nominal impedance of appr 4.2 ohms. Power handling should be sufficient since the average power of most music is but a few watts. The lower impedance will pull more current out of the amps. More current is equivalent to more signal flow and thermal stress, causing the amp to break-in both faster and more thoroughly.
When you set this up, pick out a disc that has a high average signal ( loud ) and is extended in both bass and treble response. Adjust the volume control up to a point that you would normally find yourself listening to when running your speakers and make note of this position. Shut the system down, remove your speakers from the system and connect the dummy loads. If you can find some way to elevate the dummy loads i.e. allows air-flow around all sides of the resistors, you'll be better off. Adjust the volume up to this point again and have at it. After playing a few selections, feel the resistors to see if they are warm. If you feel noticeable warmth coming from them, turn the volume down a bit. What you are looking for is to pump as much signal through the amp and dummy load without thermally stressing either of them. Just remember that if you can feel noticeable warmth from the dummy loads after a song or two, continual play will only get them hotter / have more potential for damage. As such, find a convenient spot on the volume that is both high enough to let the amp do its' thing without burning up the resistors.
Hope this helps and i've you've got more questions, fire away. Sean