I'm a little confused about how impedance from the output transformer relates to the use of an extension speaker. Take the following situation: Combo amp OT with 2,4,and 8 ohm taps Combo cabinet with 2 8 ohm speakers in parallel (4 ohm load) Main speaker jack wired to the 4 ohm tap If you wanted to use an extension cab with two additional speakers wired in parallel, What ohm rating should the speakers be? Would you want to use one of the other taps and a switch? The fender schematics for, say, an AB763, just show an additional jack wired to the main one. I'm assuming you just plug an extra cab into the jack, but that seems like it would eventually cause problems for the OT. The OT wants to see some particular impedance (in this example, 4 ohms). Doesn't plugging in another speaker (or set of speakers) cause an impedance mismatch? Is the extension jack just intended to be used when the main jack is unplugged? I'm kind of thinking the two separate pairs of speakers wired in parallel would function as two impedance loads in series. So if you have two pairs of 8 ohm speakers each wired in series, it would be equivalent to two four ohm loads in series, for a total impedance of 8 ohms, so when you're using the extension jack you should use the 8 ohm tap. Thoughts?