High Sensitvity = good transient response ?

Could you be referring to dynamic range as opposed to transient response? If that's the case, higher sensitivity speakers generally appear to sound more dynamic --or have greater dynamic range/swing than lower senstivity models.

high sensitivity usually = poor bass.

Marakenetz - is your response "high sensitivity usually = poor bass" a horns thing, or are you also referring to cone transducers as well - I am on the horns track, fwiw - and you're definitely correct there (the physics are damning)... but I've never actually scratched my head about any cone transducers claiming anything above 93db. Always given that territory to horns. An honest "I'd like to know more" here.
If anyone knows more, and has a moment, let's hear it!

It is possible to achieve good transient response and have high efficiency in a bass driver. Trying to achieve great extension AND high efficiency is a tough thing to achieve though.

The problem with most high efficiency designs in terms of bass reproduction is that they are of some type of vented design. Vents are always slower than an optimally tuned sealed box with a reasonable Q. Since sealed boxes are typically inefficient, there are few that achieve this. One exception is the Klipsch Heresy, which is sealed, has pretty quick and punchy bass ( good transient response ) and is relatively high efficiency ( 96 dB's ). Only problem is that in stock form, it is rolling off pretty hard by about 50 Hz.

Bass horns tend to add their own colouration unless phenomenally well constructed. I've never seen a mass produced model that worked really well from the factory ( in terms of "hi-fi" ). Sean
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Again, we arrive at the inevitable conclusion, every design has its flaws.
For Mwilson, I have been using Lowther fullrange drivers recently and they have 98db/1watt efficiency unloaded, 100db/1watt in a cabinet. In my cabinet, they are good down to the low 40's, and they have a resonant freq of 36, so I am not "on the spike", but rather rolling off a few db above it. This leads to a rather steep low cutoff which you can either live with, or integrate a very good sub. Right now I am living with the 40Hz bottom end, and it is quite enjoyable, although there are some records that display the flaw. But, the natural tonal quality, and speed, detail and phase coherence makes up for the 1 bottom octave of missing bass response. I know some disagree with this thinking, but few audiophile speakers actually produce much below 40Hz without some rolloff. So the sound is not really lacking much compared to most others. I have toyed with the idea of integrating a pair of TL subs of my own design, but haven't done it yet. If you have not listened to some Lowther systems, then you should really try to find some to audition. A quite remarkable full range driver.