Wasn't 600v a bit much? It lies between 55 and 30 volts, and can be measured pretty easy. You ca but a 1ohm, 2w resistor between pin 8 on the tube socket and ground, and measure milliamperes over it. U=R x I, so 1ohm times the amount of milliamperes will be the exact same amount of volt. On Marshall-amps you can find BIAS trimpots for each tube I think, if not one for all the tubes. So, find a chart which tells you how much BIAS you want, measure the BIAS, and turn the trimpot until you have the right numbers. Then you can put the tubes in it.

I don't think you can burn the amp, but you can burn tubes pretty easy. If overbiased, they are running hot, real hot, if they are underBIAS'ed, they will stay cold, and sound, bad...