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

You are indeed. And you are not alone. Whole lot of guys are missing it. Even ones who should know better. Amplifier watts are just about the least relevant spec in all of audio. 

What really does matter is speaker sensitivity. Because it requires a huge increase in power to play just a little bit louder. 3dB is a small increase, but it requires TWICE the power! Think about that. 10dB, TEN TIMES the power! 

What this means is an amplifier TWICE as powerful will only play a measly 3dB louder. No matter what speakers. That is a fact. 

So if you buy 88dB speakers, they will need 100 watts to play as loud as a 98dB speaker will with only 10 watts. 

This is the reality. Speaker manufacturers find it a whole lot easier to sell tiny little speakers, because they know women control the speaker market. Tiny little speakers are horribly inefficient so they hide this from guys they know are too whipped to do what's right by their audio anyway.

The fact of the matter is no speaker has any power requirements whatsoever. What they mean by this, their convoluted logic, is they know their speaker is so terribly inefficient nobody is gonna be happy with less than 50 watts to bring it up to a reasonable level. So they say 50. Then they also know their speaker is gonna burn out if played loud with a lot of power. So they say 50 to 200 watts. Or something like that. Point being it is all smoke and mirrors. You can safely ignore the whole thing. It is pure BS.

All you need to know is if you buy speakers with sensitivity much below 92dB you are going to start to have a hard time finding an amp you can afford they will sound good with- and the further below 92 you go the harder - and more expensive- this will be. Stay up around 95dB and above, no problem. All kinds of amps to choose from. Some of the very best amps at any price only put out around 10 to 50 watts. Which is all you need, provided only you are smart and avoid anything below about 92. All you need to know.

thanks for your response! this makes sense -  however, some literature out there also says that "low power" damages the speakers -  is there any truth to this?

Low power from the power amp or the Voltage/amperage from the power being supplied to your equipment? Lower volume should prolong the life of most any driver. Low voltage on the other hand is a death sentence for most modern equipment without some type of 120 vac maintainer... Surges are bad, brownouts are worse...

Regards

If you push an amp into clipping trying to eek out more volume from an undersized amp then you can damage a speaker. A monster sized amp means it’s still just barely ticking over at volume likely to make your ears bleed. You’re more likely to damage your hearing than your speakers with too much amp.

There are SPL meter apps you can download for your phone. Check how loud your typical listening level is. Add 10dB for dynamic headroom and calculate how much amp you need. There are calculators online to help you compensate for distance and room reinforcement. Be careful believing published specs. Sometimes they are optimistic. Also there can be adjustment for impedance and whether the spec is given as 1W or 2.83V (1W @ 8 ohm).