>i switch from ul to triode mode all the time, with 620V on the switch/screens. i can't say that i do it with any signal present though, but there is normal hum/hiss/background noise. little pop, no problem.
 
Actually, if there is some signal presnt, then the switching will be quieter than if the whole amp is muted (up to the PA input).
 
Regarding the feedback switch, even though there is a teeny dc trickle through the feedback resistor, it is insignificant compared to the DC through the ground-leg element of the loop. So, where one might expect a 'pop', there isn't one, at least not a big one. To make absolutely certain, you could put a 10M resistor across your switch. Although this still looks like an intact feedback loop, the feedback factor is zero and the amp runs wide open.
 
Have fun
Kevin O'Connor