Check my math: Calculating the size of extension cord needed
Good in theory, but... the amplifier won't draw extra current just because it's experiencing a voltage brownout. Said otherwise, its output power is likely to fall off if it's not getting fully 120VAC.

If you absolutely REQUIRE (unlikely) that it deliver 800W of output power, then you'll have to install an autotransformer at the source outlet and use it to adjust the voltage AT THAT OUTLET upwards 10.5V to 130.5VAC. Then the extension cord's drop will being that voltage right back to 120V where you want it to be.

At that distance, though... I believe I'd run no less than #12 extension cords and disregard voltage drop altogether. 200' is a long long run. #12 extension cords aren't much more expensive than #16 extension cords, and will later work for anything you can plug into them - a #16 extension cord is pretty limited for later uses.