Hi Peter
It might be worth looking at the BLA-350 circuit diagram, there maybe an attenuator before the amplifier stage.
This is certainly the case with the Yaesu FL-110 100w amp which according to the manual requires 10w of drive. I reduced the attenuation and was able to drive the amp to the full 100w with 5w.
73 Steve G4EDG
Steve G4EDG