Hazy,
I don't understand why you are tying this question to optimal power levels. I don't think it has anything to do with that.
If it is the purest tube sound that you seek, that's probably the best tube amp design with the fewest tubes, like some very expensive sets. The best tube amp design is the one with the most linear gain, lowest associated distortion levels, and lowest noise floor. The more tubes involved, most likely the harder it will be to achieve the absolute best performance in this regard. Decide how you will determine those things, then after that, factor in how it will perform with given pair of speakers, which does have everything to do with power levels, and I think that's about it.
I don't understand why you are tying this question to optimal power levels. I don't think it has anything to do with that.
If it is the purest tube sound that you seek, that's probably the best tube amp design with the fewest tubes, like some very expensive sets. The best tube amp design is the one with the most linear gain, lowest associated distortion levels, and lowest noise floor. The more tubes involved, most likely the harder it will be to achieve the absolute best performance in this regard. Decide how you will determine those things, then after that, factor in how it will perform with given pair of speakers, which does have everything to do with power levels, and I think that's about it.