With the Lowthers, you won't have any treble rolloff, unless it's happening at the amp. They are good to over 20kHz on the top end. They are more directional than some other speakers, though, on the top end, and the sweet spot is smaller than most. The bottom end, while not covering the deepest octave, is very good tonally, and it is very satisfying to hear a deeper insight into the bass player on most recordings. In some cases, it sounds better than many speakers which can play deeper, but with less finesse.
Regarding the amp issue, you can use higher power amps without worry, but you won't need to turn them up very high.
One of the main reasons that Lowthers are so popular with the SET crowd, is not just the efficiency alone, but the fact that the efficiency allows the use of these low power SET amps that are so crystal-clear, coherent, and delicately detailed sounding. It is definitely true that in a good design, a low power SET amp can be noticeably better than the push-pull amps of higher power. A SET is inherently always Class A, and never goes out of Class A. The very short signal path, few tubes, single output triode(triodes being the most linear amplifying devices known), and certain other aspects can produce an incredibly transparent, detailed, and "magic" sound, and this is why the SET users so commonly use Lowthers. Combined with the simple no-crossover "active" speaker, this makes for a beautiful combination for all but the lowest octave. But push-pull amps can sound excellent on Lowthers too, and may even have more extension on both ends, due to the configuration of the push-pull output transformer compared to the SET output transformer. Sometimes SETs can sound rolled-off on the top because of a less-than-stellar output transformer. And the push-pull may sound better in the bass as well, than some SETs. I say "some" because some SETs can really kick ass from top to bottom too.
I'm sure that the Williamson circuit amp will do really nicely with these speakers. But if you get addicted to the Lowther sound, I predict a SET amp in your future. I think you'll really be surprised at how much sound comes out of these speakers with very little power, and how lightning quick they are. They can be polite, or they can rock. They are equally at home with Mozart or Madonna. Soundstage on my system(with appropriate recordings) is about 18 feet wide, and about 10 feet deep. Imaging is pinpoint beyond what most people think of as pinpoint. I can hear the positions of every cymbal around the drum kit, are not forward of the drum kit,which is so common on many speakers. The vocalist is center-forward, and the height of the vocals is consistent with a standing singer. On certain recordings such as Madonna's "Vogue", I can hear precise imaging behind my listening position and far out to the direct sides of my ears, repeatable and pinpoint, like a surround system. In that particular recording my soundstage is about 16 feet deep, but is not confined to in front of me. It is very phase accurate. If you like to hear the miniscule details like the mouth movements and stuff, you will really like these. They are like electrostatics in the minute detail areas.
By the way, my speaker cabinets are Voigt Pipes, and are very similar to the Cain and Cain Abbeys that Philefreak talked about above, but his have Fostex and mine are Lowthers. They are both nice. I like the Lowther better, and Lowther is more efficient, and I need that with a 2 watt amp. But to each his own. The concept is virtually the same, but different manufacturers. And there are some other good single driver brands as well.
I hope that I've got your mouth watering over this description. Lowthers are kind of "love 'em or hate 'em", but I love 'em. We'll have to see where you come out. The Beauhorns are a good showcase for them. I hope he has an analog system, because that is really needed to show them in the best light. They will reveal the "digital nasties" in a major way, and it's not the fault of the speaker.
