As I understand the question, you are seeking to determine if in general it makes sense to prefer a lower powered tube amp to a higher powered tube amp (assuming that both amps can provide sufficient watts for the application), due to an expectation that the lower powered amp may, under typical conditions, tend to distort more than the higher powered amp. And thereby provide sonic characteristics that are more "tubey" and less solid state-like than would otherwise be the case.
That is a logical question, but I think it focuses MUCH too narrowly on one single factor in tube amp sonics for any answers to be useful. There are simply way too many variables involved in the design and the performance characteristics of an amplifier, and in its interaction with the technical characteristics of whatever speaker is being used, for a useful answer to be defined. Certainly among different makes of amplifiers. And even when considering amps from a given manufacturer having different power ratings, that are otherwise generally similar in design, I see no reason to expect much consistency among the product lines of different manufacturers as to whether the higher powered or lower powered amp would be preferable. Or even much consistency across a variety of speakers when a comparison is performed between a given higher powered and lower powered amp, as a given tube amp can sound quite different depending on the magnitudes and phase angles of the impedances of the speakers it may be used with, and especially on how those parameters vary as a function of frequency.
Also, the question focuses on just one kind of distortion, harmonic distortion. There are other kinds, e.g., intermodulation distortion, transient intermodulation distortion, crossover distortion, etc. Some of these are neither generally specified nor even measurable in a standardized manner. And all involve a complex signal-dependent mix of different effects and different degrees of effects at different frequencies.
What I would suggest as the most useful line of inquiry, when you reach that point, is that you start a thread indicating the particular speakers that would be used, your budget range for amplification, the room size and listening distance, some idea of the volume levels you prefer, and (especially) whether or not your listening includes recordings having particularly wide dynamic range (i.e., particularly large DIFFERENCES between the volumes of the loudest and the softest notes, such as is often the case with well engineered, minimally compressed, classical symphonic music). The latter factor will dramatically affect how much power you need. Providing that information will get you many knowledgeable and useful suggestions from the members here, from which you can develop a “short list” of candidates.
Good luck. Regards,
-- Al
