My rule of thumb is 50% -- that is, for a given speaker, you probably need 50% of the ss watts if you run tubes. It's only a worst-case rule though: if the ideal ss power is 120 wpc, you may be able to get by with 60 tubed watts. (Better to have the same powered amp in either case, it's just that high-powered tube amps tend to cost a lot to buy and run.)
When choosing a tube amp over a solid state amp, though, it is more important to consider the speaker's impedance curve (over frequency) than the amp's output. A decent solid-state amp can usually cope with a wide impedance variation. A smoother impedance curve, with a minimum point not much less than 4 ohms, will generally be easier for a tube amp to drive.
When choosing a tube amp over a solid state amp, though, it is more important to consider the speaker's impedance curve (over frequency) than the amp's output. A decent solid-state amp can usually cope with a wide impedance variation. A smoother impedance curve, with a minimum point not much less than 4 ohms, will generally be easier for a tube amp to drive.