I have the Alessandro bias meter, I have not tried the bias king.
the Alessandro is ok, easy to use but I have an issue with where / how the wires connect to the sockets. on my Fender amps, the spring retainer clips on my tube sockets push against the wires on the meter's sockets. I am concerned that the connection will fail in time.
all that being said, I have started in the habit of installing one ohm resistors in series with the cathode to ground on my fixed bias amps and have stopped using the meter on these amps, instead measuring the plate voltage, and the voltage drop across the resistor and calculating the current manually.
I have HEARD of but not experienced - some meters have the signal travel into the meter and back to the amp, possibly causing strange sounds with the meter connected - look it up, I'm not sure... possibly old marshalls ???
The Compu-bias or the Aiken Wombat (more accurate) are what you need... just pop the valves into the holders which go into the valve sockets and the little computer gives you all the figures including a simple watts output figure, which makes it easy to get to 60% bias (or whatever). You can also match valves you've got hanging around too...........