Bias refers in this context to how much current goes through a tube or transistor when no signal is present. If it is not reasonably correct for the particular tube or transistor (and the voltages it is being operated at), the relation between the output signal provided by the device and its input signal will become non-linear (i.e., not directly proportional) to some degree, which would represent a distortion.
I don't know what the repair person means by "better biasing," though. I would expect that a competent design would provide for biasing that is appropriate to that particular design, in terms of both sonics and long-term reliability (which can be compromised if bias current is set too high). And as you realize, it can be expected that the designs of higher end and lower end models in a particular manufacturer's product line will generally differ in a great many other ways that would contribute to the sonic differences you perceived.
Regards,
-- Al
I don't know what the repair person means by "better biasing," though. I would expect that a competent design would provide for biasing that is appropriate to that particular design, in terms of both sonics and long-term reliability (which can be compromised if bias current is set too high). And as you realize, it can be expected that the designs of higher end and lower end models in a particular manufacturer's product line will generally differ in a great many other ways that would contribute to the sonic differences you perceived.
Regards,
-- Al