I have dropped this link many times:
[ur]http://www.atma-sphere.com/papers/paradigm_paper2.html[/url]
It has to do with two different design, test and measurement techniques, both of which are alive and well in high end audio.
Weseixas subscribes to the Voltage paradigm which I subscribe to the Power paradigm. This gives us plenty of lively debate. I use the word 'paradigm' on account of the fact that quite often an individual who subscribes to a certain paradigm will have the opinion that anything outside that platform of thought is inherently 'wrong'. That's pretty much the definition of paradigm...
Anyway, several points. First, I did not make this stuff up. The Power paradigm existed before I was born (1956 for those keeping track) and, much like the tubes that are often at the heart of its tenets, failed to be displaced by the newer wave of transistors and constant-voltage theory that followed.
IOW it is quite possible to have bass that is not at all muddy, even with an amp that has a sizable output impedance. All you have to do is design a speaker that anticipates that, and Duke has mentioned one technique already. Now its a simple fact that all horns from the old days use these same design rules. When you try to use an amp that has constant-voltage characteristics on such a speaker, the crossover will not work correctly!! So the result is that the horn may well be subjected to frequencies it was not designed to reproduce. A lot of horns 'honk' when this happens.
IOW, horns got a bad rap on account of the fact that most of the old ones were not designed to work with transistors (in a nutshell).
Now the converse occurs when you use an amplifier with a high output impedance on a speaker that uses the Voltage rules in its design. For example, the woofer may well be seeing frequencies that the designer was trying to prevent it from seeing- perhaps an octave or two higher than it was supposed to go. This can cause the woofer to sound 'muddy'.
The bottom line here is that someone disparaging horns, amps with high impedance outputs, or planars and amps with very low outputs, **without taking these differences in design rules into account** is selling themselves short.
Now I am not an expert on the Avantgardes, but I can tell you that at least in the case of the Trio, it is one of the few horn speakers in high end that is designed to work with transistors or constant-voltage amplifiers. Consequently the task of finding a tube amp that sounds right on them is a big challenge. I suspect this is part of the reason why there are often blend problems when you try to use tubes on this speaker. So I have this advice: If you like tubes and you want a horn speaker, stay away from Avantgardes. It you already have an Avantgarde are you are trying to use tubes with it, now you know why its been so hard to find something that makes the drivers/woofers blend properly.