High Sensitvity = good transient response ?

Twl, on SET sound. I've been told they have high amounts of distortion so maybe what I've been told affects my hearing perceptions. Take "Spanish Harlem". The first time I heard on Triangle Celius and Unison SET Rebecca Pidgeon's voice sounded vaguely like if you talk through fan blades while the fan is running. Sort of funny but nice. After a short time I forgot about the "funny" sound and enjoyed the music.
You are right about Lowther being more dynamic. Brenworth's, being planars, vs. Lowthers would probably be like Martin Logan vs B&W in the dynamics department.
I heard some single driver Lowther at HE2002 and they seemed more dynamic and extending than the Brenworths. But I heard some peakiness or unpleasant edginess especially on the piano pieces played and that turned me off. Brentworth's don't do this. Sorry, I find fault in everything. But it beats the "weaknesses: none" response that pops up a lot.

CDC, the new Lowthers do not have the midrange peak anymore. They have been updated and the mids are now smooth. If you heard noise like a "fan blade" out of a SET amp, there was definitely something wrong with it. They do not normally have anything like that in their sound. Perhaps it was going unstable and oscillating for some reason, but believe me, wierd sounds are not normal for SET's. As far as distortion numbers go, they do not relate to the sound of an amp, as the measurements are not taken with music input, only steady-state tones which cannot reflect the amp's behaviour in real world conditions. This is why cheap recievers often have better "specs" than $30k amps. Whoever told you about the "high amounts of distortion in SET amps" has quite a bit to learn about audio. Those types of remarks are the sign of a rank amateur who has not yet learned much about any kind of audio designs. Avoid any more of his counsel, as he obviously is in no position to counsel anyone, since he is in dire need of counsel himself. I advise you to continue listening to various kinds of gear, as you have been doing, since it seems you have a good ear. You will be better served this way, instead of being influenced by prejudicial comments of unknowledgeable individuals. Don't worry about finding fault with everything, because everything has its faults. No design is perfect. If you know the faults, it can help you to decide which type of gear you will prefer. But use your own ears to hear the faults, and don't rely on others opinions.

CDC, according to the Brentworth website, their models use a 6.5" cone single-driver and a combination of aperiodic and transmission-line loading, called BSL. They are not planar types.

How do you make an aperiodic transmission line ? I would assume that the TL was terminated in some type of resistive ( probably stuffed ) vent.

Kind of reminds me of the Shahinian Obelisk. This is a TL that terminates into a passive radiator. Both would be somewhat out of the ordinary approaches. Then again, TL's are not very mainstream to begin with. Sean
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Sean, they don't give the details on the website, other than it is a line with "valves" to control air flow. That sounds like what you are talking about with the stuffing. I've never experimented with something like that before. It seems like an interesting concept, but I don't know how well it would work.