SET with inefficient speakers

Doesn't look like a good match, Dali recommends 50-350 wpc for the MS4. Seems to me that they designed the speaker specifically for amps of higher power.

But, for the sake of discussion, how much are you looking to spend?

The most important thing with SET amps and speaker matching is the speaker's impedance curve, much more than sensitivity. For example, an 88dB speaker with a ruler-flat impedance curve is a much easier load to drive for a SET amp than an 92dB one that dips to 1 ohm at times.

Unfortunately, Dali does not supply impedance curves for the Euphonia MS4 on their website, so it makes it hard to even guess how they would perform with SETs.

And you haven't specified if you are interested in Flea-powered(<2 wpc), low-powered(< 8wpc), or high-powered ones (20-50 wpc). There are even a few ultra-exotic amps that reach the 80-150 wpc range(NAT Se3 & Wavac HE-833Ver1.3, respectively).

One thing that might make it possible is that your MS4's are bi-wireable, so that you could drive the woofers(usually most power demanding) with a solid state or high powered amp and the tweeters/ribbons with a SET amp.

Although, there is one argument against this - the MS4's crossover to the tweeter/ribbons ~3Khz, which is little to high to get all the midrange benefits of SE triode glory.

Art Audio Jota - hi-power, hi-current version(different tubes, larger transformers). These amps can drive loads that very few SETs can even dream of. But, it will cost ya'... $13k new, ~$6.5k used

Anything worth doing, is worth doing right.

In the SET world, that usually means starting with efficient or SET-friendly speakers and building your system from there.

Any other approach, outside of luck, leads to a lot of trial & error, with most of it being on the error side.