I may be way off base here, but here goes:
http://en.wikipedia.org/wiki/Impedance_matching
Don't confuse "Impedance matching" with using a transformer in a vacuum tube power to step the output voltage down and step the output current up.
It is an impedance changing transformer, not an impedance matching transformer.
To get maximum
power efficiency out of a vacuum tube power amp you still want the output impedance to be as low as possible.
As you said, transformers are very efficient. In a vacuum tube power amp, the output impedance of the output transformer will be very low, lower than the load impedance it is driving.
The output taps on an output transformer are so the voltage and current clip at the same level into a 4, 8 or 16 ohm load.
Keep in mind that both vacuum tube power amps and SS power amps are normally voltage amplifiers (or voltage sources).
In addition, they are usually both feedback amplifiers.
Best power transfer (or most efficient power transfer) in a solid state amp occurs when the outpout impedance is as low as possible.
Don't confuse Maximum Power Transfer with Maximum efficency.
In Maximum Power Transfer efficiency can never be higher than 50%. Maximum Power Transfer: when output impedance = load impedance.
You have a valid point with horn loaded loudspeakers, because of the horn loading, the driver doesn't need a large excursion to output a lot of SPL.
The problem is, low end bandwidth is limited by the size of the woofer horn.
Modern loudspeaker are often designed for a large excursion and designed to have a high power capacity compared to older loudspekers. So you can use a long throw, high power capacity 8" woofer in a sealed enclosure in place of a short throw, low power capacity 15" woofer in an Altec Voice Of The Theatre loudspeaker.
A bit of an oversimplification, I know!
You could argue that a moden approach is to use a fairly low efficiency, long throw speaker with a high powered amp to get the same volume as a horn loaded louspeaker and a very low powered single ended vacuum tube power amp.
I wouldn't mind hearing an Altec Model 19 with a high quality vacuum tube power amp! Last time I heard one was with a crappy solid state amp. maybe that's why it sounded so bloody awful.