thank you,i was wondering about the meaning od these serial numbers

Mine is loudest when the rectifier switch in back is on number 1 labeled “Tube” and quietest when switched to 3 labeled S.S. AB2. Isn’t that backwards from what it is supposed to be? In the 2 middle position it is mid loudest too.

Just to set the record straight, the Prosonic was designed by Bruce Zinky of the Custom Shop, but none of these amps were actually built there, nor were they sold as Custom Shop amps. The “LO” prefix to the serial number indicates that the amp was made at the old Sunn Amp factory in Lake Oswego, OR. Late, production was moved to Corona, and those amps are indicated by the prefix “CR.” The Lake Oswego amps are supposed to have better-quality components than the later models, though. Fender is said to have made modifications to Zinky’s original design, but with a few circuit and tube mods, these amps go from good to really great!