My previous tube amps did not require the preamp to be connected, but if it was then it should have no signal going to the amp. What is critical for most tube amps is that the speakers are connected as they are a critical part of the output circuit. This was true any time my tube amps were powered up.
I think what you are asking is really whether signal should be playing when you are measuring and adjusting bias (" biasing should be done with no input from the preamp"). I only check bias after the amps have warmed up for at least 45 minutes with signal playing to get the amps to normal operating parameters. That means the preamp/sources are connected, but when I check bias, I mute the preamp and no signal is passing into the amps when the meter is applied. Does that make sense?
I believe what you meant to say is that amp tube biasing should be done with the speakers connected. It doesn’t matter is the preamp is on or off as long as it is not passing a signal.
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