Sorry I've disappeared; to answer your question I actually run a direct signal from my pedals to the PA before it reaches my speakers. I don't have a combo, but users of the Rumble 500 combo will be able to use an extension cabinet to move more air by using more speakers. The tone of the speakers is not going to affect the PA signal unless you mic the speakers. The head (built into the same box as the speaker cabinet on a combo) will determine how much each speaker gets based on ohms (more ohms = more resistance = less power to that cab; an 8ohm 410 will get half the wattage that a 4ohm 410 will, for example). I use two 8ohm cabinets because that allows me to put 175 watts into each individual cabinet, instead of 175 watts into only one 8ohm cabinet. Plus, stacking the two allows me to hear myself better.
Hope this clarified things!
EDIT: If you have an xlr out on your head/combo, this is taking the signal from the preamp and sending it out through XLR; the speakers on your cab will not affect the tone in the PA.