Jim, there's a little more to this story. ANY solid state RF amplifier (final) is highly dependent on what the input voltage is. For example, at 13.8 volts, an Icom IC706 will deliver 100 watts out. At 12 volts the output is about 75 to 80 watts. It is indeed possible to adjust the final drive to make up for some of the loss. However, doing so will increase the IMD well above the minimum acceptable level. With out knowing EXACTLY what setup you are using to measure the output, I wouldn't suggest you mess with the controls.
Aside from this, I suspect the controls are in the owners manual, and they are certainly in the maintenance manual. If you don't have them, the are available from Yaesu's parts department.