Man the whole underpowered-amps-damaging-speakers thing is slung around to death on forums with little or no acknowledgment of the many extremely complicated variables at play. Catastrophic amp failure aside, I think it's usually
too much power that kills speakers dead, not harmonic distortion from clipping. Still, it's important to note that an amp that's clipping can put out MORE continuous power than when playing clean...up to a THEORETICAL max of 3db (or 2x power) for a pure square wave...so if your amp and speakers are both rated for similar (honest) continuous power levels, then clipping could provide the extra power to push your tweeters over the edge.
I've read elsewhere, and agree with, the idea that if your speakers are (honestly) rated for 3-4x more power than your amp at (honest) continuous levels, then there's almost no way you're going to damage them even with heavy clipping (though I have no idea why anyone would want to listen to heavy clipping). Any more amp than that, and you're just going to have to keep a careful eye on the combination of source material level and volume control, to protect your speakers...no way around that. And the volume mark itself is no good indication of power level - it's not even a good relative measure (within the same system) because of differences in recordings. The SPL coming out of the speakers is a good relative indication - louder ALWAYS means more more power is running through.
With a vintage amp like that you have the additional concern of "catastrophic failure" due to old/out of spec parts. Get it checked up, get the electrolytic caps replaced, make sure it isn't leaking DC, etc. A full restore is always a good idea.
You can mitigate your fears by getting a nicely efficient speakers (more volume for less power) with good power handling and pairing them up with a moderately powerful (25-60 Watts), well-engineered amp