I have looked at the pictures and cannot see where the bias pot is on the photos. If it is the one to adjust the voltage between the two output valves then the answer to can it be done by ear is yes, but this is subjective. When I had a capacitor replaced on one of mine the chap that did it check the bias by putting a voltmeter across the speaker outputs and adjusting the bias pot down to as low as it would go. This is very low voltage and you need an accurate meter to do so. I did adjust mine a couple of times once by ear which is surprisingly accurate. I also did so by downloading a decimal meter onto my iPhone and placing it about 1 foot from the speaker, I then adjusted the pot until the meter registered the lowest reading, you obviously need a quiet room and house.