You can test your amp by light touching of chassis arround the input tubes with screwdriver and hear if buzz increases or you hear pops. You can also bring close light bulb. Normally BAT units will not react on light bulb brought close to amplifier but if there is a bad tube they will oscillate. That will definitely indicate that tubes need to be replaced not neccessarily input ones but any of existing.
If it won't help you'd have to get in touch with BAT and probably send your unit for service.
If it won't help you'd have to get in touch with BAT and probably send your unit for service.