For the same amount of power, the M80s will produce almost twice the sound pressure level compared to the M60s. However, the M80 will require more current than the M60. The question is, can your tube amp provide the current required for the SPL levels you are interested in? If it can function as a 25W amp for the M80s' 4 Ohm load, you will achieve 98dB or more at 9 feet. If you normally listen at 75dB, you will have more headroom than you could ever ask for.
If the amp is not rated for a 4 Ohm load, adn you constantly play it "cranked", be prepared to be changing your tubes frequently as you will be drawing excessive plate curent.
