Okay, I think I figured it out, but I just want to be sure. The two speaker jacks are both tapped off the transformer at 4 ohms and putting a 4 ohm cabinet on each of the speaker jacks does not mean the tranny is seeing one 2 ohm load (the outputs are not actually parallel to the same tap). Using one or two 4 ohm cabs does not create an under impedence mismatch. The original Fender cabs were two 8 ohm speakers in parallel (4 ohms). The amp was designed to safely drive two of these cabinets. In other words it is perfectly safe to use one or two 4 ohm cabinets on a vintage Fender amp.