They are not as robust as they could be. The zero-voltage setting is a little more critical. Most amps don't have an adjustment for this anymore. The problem with big amps is the amount of current in the power supplies. Welder is not too harsh of a term.
European parts have high quality but are hard to find in North America.
Too much bias and the amp gets hot and uses up power for nothing. Not enough and low volume sounds will be distorted while loud sounds will not sound as all that bad. Low bias can be observed on an oscilloscope as an inflection or bump at the zero voltage point of the output waveform. These resistors have very low resistance values - often 1 ohm or less. The voltage drop should be quite low - perhaps .01 to .03 volts or so.