High Sensitvity = good transient response ?

Marakanetz, perhaps you just prefer the sound of push-pull amps better than SET's. There is nothing inherently wrong with push-pull amps, and they have more power to kick the bass a little harder. My little Berning amp is actually a push-pull, using 6SN7 preamp tubes for outputs, and maybe that is why I don't feel that the bass is lacking like you say the single-endeds are. Whatever the reason, this little MicroZOTL kicks some booty on the bottom end. Really tight too.

Twl, about the 40 Hz rating. I've heard Brentworth single driver with Waytec SET (20 Hz rated) and the bass was disappointing.
Am I missing something because, take B&W Nautilus 804's for example,are rated at about 40 Hz but they seem to have more weight than the 20 Hz rated single driver? Is this a dynamics issue?

Cdc, the physics of speaker and driver design basically dictate that if you're attempting to reproduce (almost) the whole frequency range with just one driven element, you're going to have to give something up. It could be efficiency, dynamic range, extended bandwidth, optimized dispersion, etc., or a combination of some of the above. The payoff is supposed to be in the quality of "seamlessness" or "coherence" that comes from not dividing up the amp signal, not using different driver types or materials, and not intoducing lobing or phase anomolies, as well as possibly making an easier electrical load without a crossover for a low-powered amp to drive. In your above example, I'm assuming that the amps were also different, which would have something to do with what you heard (as would the rooms and setups), but yes, it's quite possible that a conventional multi-way speaker could exhibit superior bass dynamics. (BTW, what are the tolerances and conditions given [-3dB down or -6dB down? Anechoic, nearfield or in-room?] for the bass frequency response ratings you mention? If they're not the same, then you can't compare the ratings directly.)

I just checked out the Brentworth line on the web, and it seems interesting. Their specs on the top line model show a response of 15Hz-20kHZ within 3db. I find this a little hard to swallow, but they claim it. Seems they have a proprietary loading technique that supposedly allows this to happen. I've never heard them. If they sounded weak in the bass, that would be understandable, as I strongly suspect their figures. However, their idea could be a jumping off point for others to work with, and produce lower bass out of a single way system. The main question is: how do they sound in the rest of the spectrum? If they are no good there, then the bass is of little use. If they are great, then maybe I need to consider them for my system. About the comparison with the B&W's, the B&W's are a well made speaker that conservatively rates their systems, and may actually produce lower bass than their rating, in your room. In any rate, they are a good speaker, and I don't know if the Brentworths are good or not. It is notable that some single driver speaker mfr. is claiming to be able to get that kind of bass response, though. If you can tell us a little more about your impressions of the Brentworths, that would be very helpful.

Hi twl. I heard the $7,500 Type Three-A:
http://www.cadencebuilding.com/cadence/bslpix.html
The owner had the best Waytec SET amps and they are supposedly made especially for the Brentworths. I think they were over $8,000.
The room was very big - 18 feet wide x 28 feet deep (guess) so there was no room boundary reinforcement. Maybe sonic treated as well.
Listening in the nearfield these speakers are very clear. Piano was notably richer, fuller than SS gear. Had the distorted immediacy SET's are known for. I heard SET don't do deep bass and maybe with SS amp there would be more bass.
As my girlfriend said, it was all one level. Meaning there were no real highs and no real lows. Also no big dynamics you get with cone drivers. You probably know all this.
I didn't do the test frequencies but bass seemed to be running out around 70 Hz as a very loose guess. That seems really high but I just wasn't getting the base in organ music or the power in bass guitar.
These speakers had the most incredible soundstage I have ever heard. Neil Young live recordings finally made sense. On every speaker I have ever auditioned since, when I play "Needle and the Damage Done" Neil Young is pushed up above the audience. With the Brentworths, he was 20-30 feet away and the audience was understandably located around him. I didn't appreciate this feature until I could never duplicate it again. I thought Audio Physics could after hearing the Step ($3,500 model) a while back but no, the Virgo III's couldn't begin to compare. Maybe Audio Physic lost this feature because the Step's had to-the-inch listener position sensitivity and the Virgo III's sure didn't.
Hearing Michael Jackson's half speed master LP Thriller again had amazing depth and the footsteps were fast but had the SET pleasant distorted sound. This is how records should sound.
I'm not sure what else to say. They weren't for me because I like punchy, full range, dynamic sound and trying to analyze why he bought them, I guess he wants a stereo he can go home and relax with. Also he said the Audio Physics couldn't do the sounstage like the Brentworths.