i worked on an amplifier where someone tried to bias their amplifier with one of those.
it went deffective and thus caused a bias runaway to open a safety diode that saved the transformer of the unit. i replaced the diode, biased the amplifier, and the guy used the probe again to check my bias and it did the same thing.
he bought his from eurotubes.
after scratching my head on the amplifier for almost 2 weeks i finally figured out it was his bias probe he was using, the plug in section itself had fucked up.
i wholeheartedly disagree with recommending such units to anyone.
its not the units themselves or the volt-meters you plug them into. its the construction techniques of the pin connectors.
since mV = mA current ratings when measuring voltage across a 1ohm resistor, its a common sense modification that doesnt rob tone and is worthy of doing.
however, fortin's got a cool circuit he made for measuring current without breaking the circuit physically. my next goal project for my amplifier is to do the same thing. its a little tricky though. but yeah save your money and the risk guys... if it wasnt for the reversed biased zener's in that amplifier it would have been byebye power transformer.