YBA-3 is a fixed 8 ohm transformer. It does, however, perform well at 4 ohms also. But that's about it. There is a speaker out, and an extension speaker out. If you're only using one speaker out, the "ext" one won't work. Stock speaker outs are wired in series, unless previously modified. So if you want to run TWO cabs using both speaker outs, they'll need to be 4 ohms each. But if you want to run two 8 ohm cabs, you can daisy chain them and plug them in to the single speaker out, and it will perform at 4 ohms.
I believe it's 130w @ 8 ohms, and around 100w @ 4 ohms. Conservatively.
Good luck. I love my YBA-3, but I play guitar through it.