The comments by @kingdeezie are spot on. As a long time owner of A1's with the old transformers, and then updating to the newer back panels, the sound is magnificent with big tube amps. These speakers and tube amps are a match made in heaven.
I adjust the bias similar to what was described above: 1) Turn the bias pot clockwise until the crackling sound just starts; 2) Identify the setting with the hour on a clock, e.g., 11 o'clock: 3: Turn the bias counter-clockwise to the equivalent of the clock's previous hour to the one identified in step 3, i.e., 10 o'clock; 4) This process should allow you to be near the highest sensitivity without always needing to tweak the bias which is also sensitive to humidity changes throughout the year.
Hope this helps. And enjoy!
John
I adjust the bias similar to what was described above: 1) Turn the bias pot clockwise until the crackling sound just starts; 2) Identify the setting with the hour on a clock, e.g., 11 o'clock: 3: Turn the bias counter-clockwise to the equivalent of the clock's previous hour to the one identified in step 3, i.e., 10 o'clock; 4) This process should allow you to be near the highest sensitivity without always needing to tweak the bias which is also sensitive to humidity changes throughout the year.
Hope this helps. And enjoy!
John