Yes they can be a bit bright but that can be readily tamed by the tone controls.
When I got my B15NF it was red plating the output tubes, leaky capacitors were the problem. Having replaced the amps filter caps and the tubes I had several events of flash over of the GZ34/5AR4 rectifier tube. Adding a silicon rectifier, UF5408, to each plate to increase the PIV solved that permanently.
On the whole the amp is a pleasure to use and a delight to hear. Incidentally I play six stringed basses.