I use a HA3500 into a 4ohm BFB S12T - it's more than plenty loud enough.....despite folk telling me I need a GB 600/900 all the time.....I can't see the point. The HA3500's only downside is its bulk but I do have mine in the wooden box.
I used to use it with a 8ohm Hartke VX1508 and to be honest it does not seem that much louder with the barefaced, just a lot less distorted and much much clearer.
btw Alex if you read this I am loving the S12T - the more I use it the sweeter it sounds,,,,,,I am very close to my perfect tone now - very happy customer.