@jond you can keep design, execution, output transformers, power supplies, etc. the same, and simply run different tubes - 45, 2A3, and 300B, for example, to hear the differences in tube types. Yes, they may use different filament voltages, but using a power transformer that can accommodate it provides the solution.
In an SET, the 300B makes the most sense. The other tubes just don't make the kind of power to drive even the typical high efficiency loudspeaker. Larger triodes, tetrode and pentodes can also do well when used as triodes, but triode tubes of the previously listed ilk, including the 6AS7 Ralph's amplifiers employ, normally make the best sounding amplifiers.
If the product uses a push-pull configuration in a traditional transformer coupled amplifier, the number of loudspeakers it can suitably drive increases most significantly. I feel the 2A3 can drive most typical loudspeakers, and sonically outshines the 300B in these cases. The 45 may sound even better, but as it still only puts out about 5 watts in this configuration, it's still limited in what you can pair with it. Obviously, Ralph's amplifiers use the 6AS7 for logical reasons, and to very good effect