A change I did make to my unit was to remove the "Polarity' switch and it's capacitor. I then relocated the 'Standby' from the rear apron to where the 'Polarity' switch was.
I tried a couple of different drivers in the cabinet, JBL Altec, but liked the original square magnet the best. It's what's in there now.
Another change was to wire a fast recovery silicon rectifier in series with the plates/anodes of the GZ34/5AR4. This eliminated a flash over problem that reared it's ugly head. I'd recommend anyone with an original B15 to do the same mod. It increases the PIV of the rectifier system.