I think I can be of some assistance here...
1. In general when setting bias current (amps) you're usually measuring the voltage across a fixed resistance, giving you a voltage reading which corresponds to the current flowing through the bias resistor. So someone else here may be more familiar with your specific amp but if that's the case you would set your meter to measure volts and not amps. Whether you're measuring volts or amps you should choose a range that exceeds the desired setting so that an overload condition doesn't occur, although if it's a digital meter no damage would occur anyway.
2. Tube amplifiers should never be powered without a proper load on its outputs, so the speakers should be connected when setting the bias. Output transformers can be damaged if a tube amp is operated with no load on its outputs.
3. You can leave the preamp connected when setting the amp's bias, but allow the electronics sufficient time to stabilize (~20 minutes is usually fine) and then either mute the preamp or set its volume to zero before setting the bias of the power amp.
4. You should set bias with the amp connected to the outlet where it will be used. Line voltages can vary in different locations and they will affect the bias settings.
5. You can power down the amp first and then safely unplug the test cables, but if you plan on listening you can unplug the meter and then the test cable leaving the amp powered.
6. Changing input tubes should not significantly affect the bias of output tubes.
7. When an amp and tubes are new there will be some drift in bias setting as components settle in, so you can check them again in a couple of weeks after the initial setting. If the settings are relatively stable (didn't need to be reset) then you can likely run the amp for several months without the need to recheck it. You can likely go to a six month interval for checking it again, but if it is found to be drifting then you should recheck it again sooner. No difference in checking or setting bias, you keep the speakers connected and make measurements with the amps warmed up and preamp volume muted or at zero.
Hope this helps and welcome to Audio Aficionado!
