Speaker wants 100 watt RMS. Is 40 watt tube OK?

otherones, depends on what you like listening to and how loud. If you are a stadium  rocker forget it. Jazz, classical and light rock at moderate volumes no problem. Personally, I think you are making a mistake. Save a bit longer and get a more powerful tube amp if you have to have a tube amp. 

Ripple

That idea that 3 dB is a very slight change in volume is ludicrous. 1 dB is a slight change. Raise one channel 3 dB and the other channel will disappear. The image will shift all the way to the louder side. 
Damping factor only matters at very low frequencies below 100 Hz.
Back EMF is highly overrated. Yes the speaker is a generator but a weak one. Only very large subwoofers with huge magnets might present a problem but the SS amp is protected by it's output stage's very low impedance and the tube amp by its transformer.
Power is power. Tube amps my clip more gracefully but any clipping is distortion. 
At any rate subwoofers require a high power amp with a high damping factor (low output stage impedance) Above you can pick your poison and power is not as critical. 

celtic66, every recording has a "right" volume. And that right volume is multi-factorial. It depends on the type of music, the way it was recorded and mastered, the way your own system is equalized and the way our ears change frequency response with volume. Bass and treble increase with volume (see Fletcher-Munson). A recording that was meant to be listened to at loud volumes will sound dull and lifeless at low volumes. Turn it up and it sounds great but if you tried to play a string quartet that loud the violins would slay your ear drums. Rickie Lee Jones albums were recorded on the bright side and sound best (more natural) at moderate levels. Turn it up and her voice does that violin thing. I have an old Buddy Miles album with some great songs but it always sounded lifeless to me. I hadn't played it probably in decades. At 95-100 dB the record is a knock out. They were probably stoned and had the monitors turned up to the max when they mastered it. 
A word about listening to loud music. Always warm your ears up. In your middle ear you have a muscle called the Stapedius which dampens the Stapes bone keeping it from vibrating as hard against the oval window. Google it and you can see the anatomy. When exposed to loud sounds your brain tightens the Stapedius through the 7th cranial nerve lessening the amplitude the inner ear sees (hears) but it takes a little while to tighten. Don't just go right to 100 dB. Start off at 85 dB and crank it up 5 dB every few minutes till you get where you want. Beyond 100 dB is no man's land. It also takes a while for the Stapedius to let go which is why after a loud concert you can't hear anything for a while.
celtic66 quantity and quality are essentially the same or so closely tied that you can not separate them. The quality is not there unless the recording is played at the right volume which is determined by all those factors above.