The answer is dependent partially on the design of the speaker. If the speaker is a Maggie or Soundlab, the answer would be different from a horn or cone speaker. The reason is that some speakers require more voltage than wattage, and some require more damping and/or control. You must decide what type, design and brand of speaker, and then look for the amp. I have owned solid state and tube amps in all price ranges, and some combinations are created in heaven and some (unfortunately) in the other place. The argument between solid state and tube has a great deal to do with personal preference and the experiences of the user. Most people don't argue or cling to a belief to be difficult. Most are certain of their decision based on their own sound system and what has worked for them. We are all after "perfect" sound, that cannot be obtained.
how much tube power is needed?
Let's say, for a 86 and a 92 dB efficiency speaker. SE triode fans say 10 W is enough, 20 W is more than you need. They use horns and high-efficiency speakers (> 92 dB sensibility). They say high-powered designs do not sound good at low volumes while driving high-efficiency speakers. Others (mainly push-pull fans) say that even though you have high-sensitivity speakers, the more watts, the better dynamic resolution. I don't want to launch a SE/PP war now. This is not meant to be a pure technical question, it also concerns musical taste. What is your experience with these?