I can’t speak for those who play speakers at extremely loud levels. If I had speakers as efficient as yours, I would not rule out amps rated as low as 5 watts per channel. This brings into play low-powered tube amps (my favorite kind of amp), or some low-powered solid state amps, like those offered by First Watt (Pass Labs offshoot).
The thing with tube amps is that it is very hard to find a good one for really low prices because the cost of decent output transformers alone can be quite high. Used amps can keep the price down and are a good option with tube gear because even very old amps tend to be serviceable (many solid state amps are harder to repair because they tend to be more complicated and have parts that become unobtainable).
With speakers of this high efficiency, I would be looking at tube amps running EL 84’s, 6L6, KT 66 (pentode/tetrode pushpull amps), or single-ended triode amps. Because of their simplicity, some might expect single-ended triode amps to be inexpensive, the demands placed on the output transformer means that decent amps can be quite high in price. The cheapest decent one that I know of is a 300B amp from Elekit (you save a lot because it is a kit that has to assembled), but the other issue is that the 300B tube can be quite high in price.