20 year user here as well. All over the world with it. Here is the manual link. You are giving the mosfets more juice when you crank it to 10. This is what 'acts' like a tube driven power section. More voltage to a tube makes the tube work harder. The harder the tube is driven the more characteristics of a tube will be present. What is confusing it Ampegs words by using descriptors like "thickened" and "compressed" as those are usually reserved for tube characteristics not the other way around. http://www.ampeg.com/pdf/svt-3pro.pdf
From Section 113:
"The tube gain control varies the high voltage supply to the preamp tubes. This allows a variety of characteristics from the power amp and replaces the limiter found on typical SS power amps. At '10' the voltage is at maximum, providing a dynamic, highly responsive tone. At '0' the voltage is at a minimum , offering a thickened, more compressed tone. This tone may also be distorted, depending on the volume level. In between settings are best for preventing harsh distortion when driving the power amp to its limits. The effect of this control increases from moderate to dramatic as the power amp is driven harder."
But I am probably just reading it wrong.