The problem with setting the Bias and not checking the Plate Voltage is this:
The Bias Current is supposed to be set at a certain ratio to the Plate Voltage to achieve 70% Plate Dissipation, give or take 5% or so.
It's not exact, and here's why:
Every set of tubes you get, unless they have been tested and fall within a specific range of current draw, will have a different effect on the amp.
Mesa (and the guys who sell Mesa replacement kits - most of them anyway) run their tubes through a 'Characterizer' or other machine that tells them what the current draw, plate voltage, bias current will be for a given situation.
This is how they can sell you a set of tubes that are simply 'Plug-n-Play.'
Unfortunately, I really don't like the tones that current Mesa power tubes have to offer and I don't think I should pay that much for tubes, not even NOS.
I use this tool: https://taweber.powweb.com/biasrite/br_page.htm
That way I can make sure that the tubes are drawing the proper amount of current.
To me, it's really important to check the Plate Voltages several times while biasing the amp. They will change as you change the Bias Current.
As for tonal differences between a properly biased amp and one that is biased poorly...
I find that the amp feels more alive when you play it. Also, it removes a lot of the 'Blanket' that some people hear. And, it seems to increase the available headroom if the amp is biased properly.
I think one of the main reasons that Mesa biases their amps on the cold side is to make sure that the tubes last longer.
They are in the business of selling reliable, kick *** amps. Last thing they want is their customers saying, 'It sounds great, but man does it eat tubes...'
