That is not correct. Doubling the drivers, given the same output that a tube head delivers, results in a theoretical +3dB. Other things happen with doubling drivers as well. Due to coupling, lower frequencies benefit more, so there will be added low end response as well. That is why the standard recommendation of how to get more perceived volume is to double the drivers instead of doubling the power. Now if the user was using an SS head, and doubling the drivers created the scenario of going from a 8ohm load to a 4ohm load, you can pick up an additional 2~3 dBs depending on the specifics of the head for a net increase of 5~6dB.