Please be very careful while checking this, it is high voltage.

So, when you put your red probe on the Standby switch's solder lug connected to Pin 8 of the rectifier...and your black probe to Ground...you get 400VDC?

And when you put your red probe on the Standby switch's solder lug that connects to point "A" on the layour...and your black probe to ground...you get 100VDC?

You haven't even reached the first filter caps?

If this is the case...there is only three things inbetween..the switch, its resistor and its cap....try jumping the switch lugs to bypass the Standby switch.  Now does your 400VDC pass on to the filter caps and choke?

Here is your schematic, it's what you need to trace everything through to troubleshoot...layouts are for construction.
http://www.amparchives.com/Amp%20Archives/Fender/Schematics/bassman-5f6-a-schematic.gif