David12, the difference between 10 watts and 20 watts is 3 db. That's not a lot to the human ear- something else is going on.
SETs make very low distortion at low power levels. Once they get over about 25% of full power, then a substantial amount of harmonic distortion comes into play- including the odd ordered harmonics. Now music is composed of transients which consume most of the power, with less power in-between the transients (unless you listen to pipe organ). In addition, one of the most important human hearing rules is that the ear is uses the odd ordered harmonics to determine how loud a sound is.
The confluence of these two facts is that you have the loudness cues on the transients when using an SET that is being asked to make too much power (over about 25% of full power). This is why SETs have a reputation for being far more dynamic than their small power would suggest. If you get a sound pressure meter and actually measure how loud you are playing the system, it won't be all that loud, even though is *sounds* like it is loud, due to the artificial loudness cues.
IOW an SET running on a speaker of moderate efficiency (91 to 95 db or so) will sound louder than it really is. If you really want to hear what an SET can do, you simply have to have more speaker efficiency. In the old days (before my time, nice to say that for a change...) when SETs were the only game in town, *all* speakers were high efficiency.
Believe it or not, a stereo should not sound loud. IMO it is the mark of a good system that it remains relaxed at all volumes, even 105db or more. It should not drive you out of the room!
You may think you are doing OK with a lot of the moderate efficiency speakers mentioned, but you can be assured that you are not hearing the actual magic (inner detail) that SETs are capable of. I had ZU speakers for quite a long time, and have Classic Audio Loudspeakers now, both are about 98 db. I find that 60 watts is about right as it is nearly impossible to clip the amps. I have 2A3, type 45 and 300b amps; they can play the speakers certainly, but they fall right on their respective faces if I try to push them.
I will agree that there could be a listener preference going on; I like things to sound relaxed, but at the same time I play a lot of rock and electronic music that can be pretty demanding (I'm also a bit of a Wagner fan and some of the Decca-recorded operas are pretty demanding too).