I was so close to buying this off Andy! But I finalised buying an SWR Big Ben hours before Andy got back to me.
I have the Big Ben (8 ohms) under a Super Redhead and at a total of 2.6 ohms it sounds awesome....THEREFORE.. the Son of Bertha will work really well! The Redhead specs say only a 50w jump in output between 4ohms and 2.6ohms and 50w between 2.6 and 2! You wouldn't notice the difference