The success of the Pass 25 and other small-bore tube amps Indicates that's plenty for most applications. But remember this - speakers are current driven devices and benefit greatly from amps acknowledging that fact.
Most modern speakers are less than 8 ohms and so are quite current hungry. Also, loudness is logarithmic, so twice the power is only 3dB louder - a just noticeable difference. To sound twice as loud requires 10X the power. Also remember the limits of your speakers. KEF LS50s 84dB @ 1W/1M and 100W limits means they are limited to 104dB maximum output. A big JBL 4367 is at least 10dB more efficient and will handle 200W peaks for a max output of 117dB. The KEFs are meant for near field listening only a few feet away, the JBLs meant for a large room with the listener 10-15 feet away, so it all must be worked as a system, not just components in isolation. As for me, my 87dB Monitor Audio S300 7G and a solid 60W amp in my 12X16 room is plenty. In other systems, 200W was just adequate, and in another a 40W tube Mcintosh MC240 was fabulous. Lots of valid answers, just don't ignore physics.