wattage difference between SS and Tube

I'm not sure how efficient your Thiels are (I don't think THiels are usually all that efficient), and efficiency makes all the difference. For some speakers, 50watts is two or three times more than they could ever use...for others, it's only 1/10th of what they need. Ask Thiel what the power requirements are for your speakers and ask if they recommend any particular types of amps. As far as wattage and amperage goes, a 50watt tube amp should sound more powerful than a 50watt SS amp (someone else can explain "why", because I'm not electrically inclined). Volume isn't always the issue with wattage, because wattage and volume aren't a one to one ratio. I believe it's a ten to one ratio, so you need to have a 500 watt amp to be able to play twice as loud as a 50 watt amp (everything else being equal).

Smaller Thiels can usually get by with 50 watts but benefit from more. Thiels do present a rather linear but low impedance. This would be the major concern IMO. Power requirements can be crudely determined by speaker sensitivity, room size (and construction) and volume sought. IMO it's harder to get an amp with high power to sound good than a lower power amp to sound good, but the benefits are worth it. A very crude (!) rule of thumb as far as power recommendations is to double the manufacturers minimum recommendation. I suggest being as mindfull of Thiels low impedance as much as to the sonic signature of the proposed amplification.

Tube 50W "screams" more pleasant than SS 50W. On the lower volume levels they will sound absolutely the same in terms of loudness. The low (10x less than transistor's) output resistance of the tube allowes to draw the maximum current onto the speakers but needs to be matched with the speaker's impedance by means of matching output X-former. Thus even "under-powered" tube amps(i.e. bellow speaker power requirements) can successfully control the driver's coils. On my experience, low-powered SS amps(unless it's Pass Labs Aleph series) can't controll speakers. The beat of drum continues longer, bass isn't tight and deep.

SS designes are soffisticated and require a large amount of negative feedback(upto 30...40dB) to bring the circuit elements(transistors) onto linear operation on the audiable freequencies decreasing dynamic performance that is so necessary for orchestral music. Bringing SS amps to the peak powers will certainly drammatically change the output freequency bandwidth curve and the amplitude will reach non-linear area of output Volt-Amp characteristics(clipping)

Since tubes have linear output Volt-Amp characteristics its designes use minimalistic circuits and very often with no feedback(but for real feedback is needed to increase the freequency bandwidth). The signal is more transperent and open when the circuit path is minimal.