I am not surprised that people report good results with SET 2a3 and 300b amps. I generally do find that most people overestimate the amount of power they really need. For the vast majority of time, one listens at levels where a watt or two is enough.
But, there will be some limitations with these types of amps and speakers in this efficiency range. With really demanding music, such as large-scale choral music, at crescendi, one will hear the effects of compression--the music stops getting louder and it becomes more muddled sounding. I hear this to some extent when using my 5.5 watt amps on 99 db/w efficient speakers. It is not a big deal to me; I don't attempt to listen to Mahler's 8th at "realistic" volume, but it may be a bigger deal to someone else. This is something everyone has to judge for themselves.