"In theory," fewer tubes mean less concern with mismatched tubes, which means less muddled sound. A lot of purist won't even except two output tubes per channel in a parallel single-ended setup for this reason.
But, in practice, there are quite a few advantages to more tubes to achieve a given output. One advantage is not having to run the tubes as hard. When run well under their rated output, most tubes will last MUCH longer, and this may actually mean lower cost for tube replacement than another amp that uses few tubes but pushes them much harder. Using multiple tubes in parallel lowers the output impedance meaning a lower turn ratio for the output transformer (arguably a positive result), or in the case of OTL amps, a lower output impedance seen by the attached speaker (higher damping factor).
One of my all-time favorite amps is a custom-built OTL amp that uses many tubes to achieve a 35 watt output. The tubes in that amp, which is often left on for months at a time, have not been changed in more than ten years and still test new.
But, in practice, there are quite a few advantages to more tubes to achieve a given output. One advantage is not having to run the tubes as hard. When run well under their rated output, most tubes will last MUCH longer, and this may actually mean lower cost for tube replacement than another amp that uses few tubes but pushes them much harder. Using multiple tubes in parallel lowers the output impedance meaning a lower turn ratio for the output transformer (arguably a positive result), or in the case of OTL amps, a lower output impedance seen by the attached speaker (higher damping factor).
One of my all-time favorite amps is a custom-built OTL amp that uses many tubes to achieve a 35 watt output. The tubes in that amp, which is often left on for months at a time, have not been changed in more than ten years and still test new.