My 91db speakers are driven just fine by my 20W tube amp. (I will say, though, that with a similarly-priced SS amp at 40W, they sounded like sh*t.). I doubt that there's a general rule that you could follow that would trump the judgment: "What does it sound like? Do I like what it sounds like?"
how can low watt tube amps drive speakers with higher power requirements
I am new to hifi and I am super confused about something. Most audio blogs out there ask newbies to stick to amps that output power within the recommended range of the speaker manufacturers. However, on forums, blogs and even some magazine articles, I find pros reviewing tube amps with much lower output power (even in some cases 10-30W below the speaker specs) and find no problems. How can these low power tube amp drive these speakers? For example, the LS 50 metas spec sheet says "Recommended amp power: 40W - 100W) but I have seen posts here and on other forums where people will hook these up to tube amps producing as low as 12W of power at 8 ohms. Am I missing something?