Watts are watts. Tubes sound better than SS when they clip and its more subtractive. SS just sounds crappy when they clip.
Some manufacturers of solid-state amps have taken this into account. Levinson amps, for example, have a built-in smoothing circuit that eliminate this harsh clipping sound when the signal gets close to the supply rails. Another that comes to mind is McIntosh, whose PowerGuard circuitry has received acclaim as an effective mean to end hard clipping.
As for the Watts question, in theory it shouldn't really matter if the amplifier is tube or solid-state, because strictly speaking 1 Watt = 1 Volt x 1 Amp. In practice, the specification is approximate, so you can expect an amp with a bigger power supply and lower impedance to sound louder with the Thiels. Also, all amplifiers differ in their "power margin" above its nominal power, depending on their circuitry details.
As a general rule, 100 Watts per channel would be a good starting point to choose an amp for the Thiels. If the brand is serious it will correctly specify its product, so you shouldn't find differences of more than 1 o 2 dB in acoustical output between a solid-state amp and a tube amp in this particular case.