Skylab is correct. You can't generalize the amp-speaker matching with a blanket statement like you need 25 wpc at least not to clip, even without considering sensitivity/impedance of the speakers. It would be a ridiculous task to drive Skylab's B&Ws, for example, with some 300B SET amp putting out 8 wpc. But with 98db+ sensitivity speakers with flat impedance, I assure you the volume will be loud enough to drive you out of the room. As I said, my Harbeths are not the ideal mates for the Leben, at 86 db sensitivity, but due to its easy load (flat impedance), I listen to my music plenty loud enough for my tastes, can do Wagner and the early Kinks without any need to apologize...
The reason why some tube aficionados also insist on fewer watts, is that they believe that it's the first couple of watts that are the most critical to getting that pure musical sound. This is a philosophy that is embraced by a lot of SS manufacturers, too. For example. My Plinius runs pure Class A for the first 2-3 watts, then kicks into AB, as do a lot of other quality amps I've seen. Again, there's always a trade-off... with SS, you do get better bass control & resolution in general, but IME, flatter soundstage and musical sound that is less unbelievable, in comparison with live music. This is because with a SET amp, for example, it leaves in even and odd ordered harmonics, which helps in the simulation of hearing 'live' sound. SS amps do not do this. Some tube guys also disdain push-pull amps b/c they leave out even-ordered harmonics. SS campers would call these harmonic orders 'distortions.' It's ultimately a subjective judgment, but let's think about how we hear live music. These harmonics are present in every environment in which we hear music live, as music is experienced organically, communally, in halls or stadiums or cafes, as opposed to in clinical environments.