if you're speaking of tube amps, then it's 400+vdc vs 9vdc . . . the former can kill . . .
I should've clarified myself... I meant as far as tweaking circuits and such -- you can't really breadboard a 400 v circuit, correct? I'm really getting the itch...
Yes, for tube amps, most of the "tweaking" must be done on paper. Then build it and perhaps play around with different caps.
I find it pretty safe to work with tube circuitry, because I always double, triple and even pentacheck everything before powering up. And while flipping the switch, I usually take cover behind a sofa, in case any caps decide to go bye-bye.
If you don't care too much about reading the following link, you might aswell stick with your nine volts. http://www.drifteramps.com/safety.html
Otherwise, welcome to the tube family.