low power tube amp

Hi klee123,
The potential problem would be driving the lower power amplifier into the range of clipping which could damage the tweeter. So it depends on how loud you listen and how hard you’re pushing amplifier.

Jperry raises pertinent questions, as there’s such variation among audio components. Two amplifiers rated at the same 20 watts can in reality be virtually night and day different as to their performance. The quality of their design, implementation, transformers and power supply could place them at polar ends of a spectrum. This is not exaggeration to simply make a point. Other than the same power rating (watts)there may be no other common ground.

If possible I’d listen to the pairing and judge based on what you hear. Depending on your listening volume and music genre you may be using just a few watts the vast majority of the time. Power is an important consideration but so is the overall  quality of the amplifier.
Charles

It's a serious problem, don't do it. A good chance you will damage the speakers when you clip the amp. If you absolutely love the speakers, then get a more powerful amp, but I'd suggest finding speakers that are made to mate with your lower powered amp. Cheers,
Spencer

A tube amp will clip "softly" by rounding off the wave, as opposed to the much more dangerous hard clipping produced by a transistor amp that's pushed too hard.  My speakers are recommended for 150-800W, and I've driven them for years with a 38-watt tube amp.  If I can remember not to be greedy by chasing slam and impact by turning up the volume knob, I'm rewarded with stellar imaging, tone, and texture.  Going too loud flattens the imaging and transient incisiveness.

You won't get pants-flapping bass and room pressurization out of your SET, but if that's not the highest priority you may still be pleased with it.  Hook 'em up and listen.

Soft clipping tube amp is less likely to damage speakers when clipping. 

Cymbop,
Your 38 watt amplifier is able to do what it does no doubt based on high quality power supply design and transformers. Another (lesser quality) 38 watt amplifier could fail miserably with these same speakers. As you said, hook them up and just listen.
Charles