So I've been using this 12" combo amp for the past 2 years and it's been amazing so far... amazing tone, lightweight, no problems. I'm finally thinking about adding an extension cab of some kind, possibly a single 12" or 15", but I'm unsure of the whole impedance thing. Unless otherwise requested, all the Walkabout Scouts that leave the Mesa factory are 4ohms, allowing you to get the full 300w of power from the detachable head, so I'm assuming I have the 4 ohm version (I bought mine used, and I didn't think to ask the guy I bought it from at the time). So if I did get another cab, what impedance should it be assuming mine is already 4ohms? I'm kind of a noob with this whole ohms thing, but I know that if I add another 4 ohm cab the total impedance will be 2ohms, but if I add an 8ohm cab the total impedance will be 2.67ohms. Should I play it safe and go with the 8ohms?