If two coherent periodic waves are combined, the combined level will be 6 db higher.
If you're running a tube amp and adjusting impedance to properly match the combined speaker load, then the output of your amp will be be the same. Put the same amount of power into two speakers instead of one, and each speaker will see 3db less power.
So you lose 3db power by running two speakers, but gain between 3db and 6db SPL. Therefore, the total SPL gain is between 0db (none) and 3db (a noticeable volume increase).