Maybe, like various other cabs, it's switchable between 4 and 8 ohms. Enabling full power into a single 4 ohm load??????? I dunno
I take the 4 or 8 ohm switch to only affect the external cabinet, allowing for it (them) to total 4 or 8 ohms, without changing the internal speaker impedance. I also assume the internal load to be 8 ohms outputing "only" 500 watts without any extension(s), like the Bassman 800 into a single 8 ohm cab. Of course, I could be wrong about all that.
It's the operation of the 4 ohm switch, requiring a 500 watt rated cab, that I'm really guessing at! Like @pudgychef says, it seems to be stable at 2,67 ohms to allow a 4 ohm external load along with a 8 ohm internal load. How it do dat?
