well it's all about the combination of speaker impedance curve/phase angle and sensitivity and efficiency of the loudspeaker - it's a complex relationship - in the real world you have to just try it and see (or hear) how good it works, paper specs only a guideline.

with a valve amp using output transformers it's even more of a lottery since you can get frequency response variations as it interacts with speaker impedance. This could make a valve amp seem more powerful depending on that interaction.

I'm personally not much for valve power amps (unless OTL) since you really do need to choose the speaker very carefully to get an accurate response and for me that's the tail wagging the dog. I rather choose speaker then get an amp to drive them.

Heard too many systems with valve power amps and you get that bloat or even boom in the bass which ruins the whole sound - although some seem to like 1950s radiogram bass.