how can low watt tube amps drive speakers with higher power requirements

I build my own custom tube amps for sale and for my own use. My favorite  design is a pair of NOS 300B tubes operated in single-ended, ultra-linear mode. About 15 watts RMS output. It drives even the most power-hungry speakers st acceptable volumes for listening. It won’t; of course, blow the doors off your listening room, but; really, who needs that kind of output unless your half-deaf or a heavy metal freak?

They don’t. They clip at the slightest power chord.  

I agree 100% with millercarbon. An audio system is something that takes energy and turns it into something useful. Think of it like an internal combustion engine, which is basically an air pump. The more air it can take in, the more air it can put out. That equates to horse power. Restrict the input and you restrict the output. Give it plenty of input with a supercharger but restrict the out put with a crappy exhaust system, and you are just defeating the whole system. That's what choosing low efficiency, hard to drive speakers does to an audio system. Choosing low efficiency speakers is just stacking the deck against better sound in an audio system. Sure, a tube amp can play with low watts and low efficiency speakers, as long as you are content with low volume. The first time you go to crank up the volume, you'll discover the error of your ways in the name of distortion. Great sound is all about speed and reserve power. A system that is fighting it's speakers has to give up speed, and that equates directly to quality of reproduction, because a reserve of power to keep up with the rapidly changing dynamics of music is what makes an audio system truly shine. Even with high wattage systems, low efficiency speakers make no sense. Your speakers have the final say in how your system sounds. Why allow them to be a road block? As miller aptly stated the true and most important spec of a good speaker is it's sensitivity, and, the higher, the better. 

It is all based on the quality of the first watt with a lot of amplifiers and speakers and the ability of the amplifier to drive the speaker. The other main factors are room size and listening level wanted. Small amplifiers can be good for a lot of music and speakers but only at low listening levels turn up the volume much at all and you will tax the small amp and you weigh the choice between volume and quality of sound with small amps. The key is to find a good all around amplifier that allows you to play whatever you want on any speaker at any volume, they do exist, but are hard to find.

A low powered tube amp is unlikely to damage a tweeter if driven to distortion, it simply sounds bad.