I have never really fell in love with the sound of any system that was driven by high-powered amps of any sort, solid state or tube. With high-powered solid state, I just don't stay as engaged with the music because there is a lifeless quality which means turning up the volume louder to get the juices going. With high-powered tube gear, there is a "glare" (sense of a sharp peak) in the upper midrange/treble and a brittle quality I don't like. For me, it is lower powered pushpull or SET or OTL's that do the trick. That does mean some limitations on speaker choices. I have not owned Harbeths, but I do hear them a lot and with many different tube amps so I know they can be a challenge. The 3,7, and 30.1 can be used with medium and fairly low-powered amps. The temptation with the 5 and 40.2, because they are bigger speakers, is to play them at high volume levels. I suppose having plenty of power helps for that kind of listening. Because I don't like high volume listening, I do not think these speakers are necessarily inappropriate for use with lower-powered tube gear. The best I've heard from both of these speakers was with a 10 watt pushpull amp. I also liked the 40.2's with a 40 watt parallel SET amp. I actually liked the 40.2's less well with the 150 watt pushpull amp and another pushpull amp that was also in the same power range. Harbeths are no different from any other high quality speaker. They are easily capable of showing differences in upstream components, and in that sense, they are sensitive to choices made. But, being inherently on the warm and musical side, they are also somewhat forgiving of upstream components--they will sound good no matter what they are coupled with, even if they are not at their best.