How do tube Watts compare to SS Watts?

Mathematical formula is as follows...
Tube watts = (SS watts) X 1
.

Your real question should be "Can I drive my Theil 2.2's with a modest sized tube amp?"

There is a great deal of interaction between an amp and a speaker. A speaker with minimal impedence drooping to or below 4 ohms are not good candidates for anything much less than a high power tube amp. A speaker with an impedence curve that has big swings (as opposed to being relatively flat) may not be good with most any tube amp.

But apart from that, assuming that your speakers are any easy load with an 86db efficiency I'd want at least 80 watts with tubes and 100 to 200 with most SS unless they were really high current amps.

Watts are watts. Tubes sound better than SS when they clip and its more subtractive. SS just sounds crappy when they clip.

The size of the room factors into it as well as the impedence curves that Newbee mentions. Looking at the Stereophile measurements, the impedence curve of your 2.2's is relatively flat, but a lowish 4 ohm load (3.5 min, 5.2 ohm max).
I would think that you may get by with a 50-100 wpc tube amp in a smaller room.
Overall though, I think you would be better served with a tube preamp and a SS amp, or a SS integrated amp.

Watts are watts, and I second Newbee's comment that tube amps will clip more gracefully.

I have used a pair of Wright Sound WPA 3.5 single-ended triodes with Thiel 3.6's. That's a whopping 4 watts into a 6 ohm, 86 db per 2.83V/m speaker. While it doesn't do justice to orchestral, opera or rock, it certainly played loud enough in a 27 X 16 room with classical (chamber, solo, instrumental and voice) and jazz.

Your mileage may vary depending on your room, what your sources do and what you consider adequate playback volume.