Interesting. Never heard of this. It would be difficult to mount this "mod" on a switch though, so a subtle change would be hard to notice. Do you have any idea what actually happened electronically? I have the impression that the 5f6a Bassman does not like too much preamp gain.
Don't think I'm educated enough to explain it well scientifically (perhaps Wally can), but the 12AU7 has a lot less gain than a 12AX7 but a lot more current than a 12AX7. This provides a stronger and smoother push of the signal thru the EQ section before hitting the phase inverter and power section. I really like it.