Most high efficiency speakers have fairly tight tolerances in the voice coil gap which makes them very reactive. The back EMF they present to amplifiers that use a lot of feedback is enough to confound the amplifier, as the feedback signal thus contains induced errors.
This is why transistors in general tend to sound shrill on horns and why horns had such a difficult road back into high end audio in the last 20 years- the bad rap of a bad combination.
However if not so much a tube/transistor thing as it is the amount of feedback used by the amplifier. Transistors do tend to use a lot more than tubes, and there are tube amps that don't use any. There are some transistor amps that don't any feedback also and not surprisingly they don't do so bad on horns.
I heard the new field-coil Shindo speaker at THE Show. It seemed to have some potential (no pun intended) but was clearly not playing the bass that you would expect out of a driver and cabinet that large. The material I played has information in the mid-20s, and my suspicion is that the Ongaku amplifiers that were being asked to play the bottom end were simply not up to the task, but its only a suspicion. When hearing any system there is always the tendency to place the blame of shortcomings where your biases lie... regardless, I think that the speaker they showed is really something to watch.
I'd really like to see how it stacks up against the Classic Audio stuff- they cost twice as much as the Classic Audio speakers do.
