A 9 watts beats Passlabs, EAR and YBA ?

Its all about the rules of human hearing. Negative feedback generates distortion that the human ear is very sensitive to: odd ordered harmonics. They cannot be eliminated unless you also get rid of negative feedback! So amps that are zero feedback will often sound inherently smoother and more musical than those that use it. more:

http://www.atma-sphere.com/papers/paradigm_paper2.html

Hi Eldartford, sure. All amplifiers have what is called Propagation Delay, which is the period of time that it takes a signal to propagate from input to output. This propagation is caused by the risetimes of various internal stages and is not 'the speed of light' which is a common misconception.

In fact the propagation delay of any audio amplifier can be easily measured on any decent oscilloscope.

The propagation delay is a constant; it is not a problem unless there is negative feedback. Lower frequencies that are fed back tend to arrive much closer to the actual time that they need to be there (although the propagation delay means that they will *always* be a little late). As frequency increases, this delay has a more profound effect, essentially introducing a ringing effect.

This ringing is easy to understand if you think about a positive-going pulse of short duration being applied to the input. The amplifier responds to it and the feedback applies an inverted version of the pulse at a lower amplitude back to the input. By this time the original pulse is either gone, or nearly ended. Now the amplifier has to amplify the negative-going pulse; this process goes on as a ringing phenomena that dies out, although wider-bandwidth amplifiers often have to have filters in the feedback network to prevent this phenomena from causing oscillation, particularly if the amplifier exhibits phase shift at higher frequencies!

The result is low-level odd-order harmonic generation, usually the 5th, 7th and 9th are the greatest concern. It does not take a lot- the human ear so sensitive to odd-ordered harmonics that hundredths of a percent will audible (as a hardness or harshness) when a band of frequencies is being amplified.

Conversely, the human ear does not object so much to even-ordered harmonics. These create 'warmth' and 'lushness' but are a coloration nonetheless. Audiophiles have words to describe harmonic distortions of both types.
odd ordered descriptive terms: hard, harsh, clinical, overly-detailed, chalky or chalky-white, brittle, etc.

terms for even-ordered harmonics:
warmth, bloom, lush, fat, muddy
-as distortion increases.

Hi, Eldartford, the idea that you can run zero feedback at some audio frequencies and then use feedback at low frequencies suggests to me a gain non-linearity that changes with frequency. So that would take some tinkering to make that work, to say the least. To my knowledge no-one does this, as the idea behind many amps that use feedback is the concept of 'constant voltage' wherein the amp is capable of constant voltage regardless of the load impedance.

Zero feedback amps do not behave this way for the most part, usually subscribing to a different idea wherein the amp makes 'constant power' with respect to the load.

These two ideas have been in conflict in audio for the last 50-60 years, resulting in the tube/transistor debate and the subjectivist/objectivist debate, plus the general idea of component matching.

Robertwolfee, the Pass Labs amplifier is class A, but it is not tube. However it is one of the best-sounding transistor amplifiers out there. At any rate you will need some power for the Maggies, so due to the power limitations of most SETs, you won't get them to be very practical with your speakers although I am sure they they would sound fine, just at very low volume levels.

Eldartford, you mean, 'How come the propagation delay in an amplifier causes odd-ordered enhancement and not even ordered?'

To answer this I think you have to look at the components of a square wave -odd ordered harmonics- and then look at what happens when you add a delayed inverted signal to the original signal. The result has a bit in common with what happens to a sine wave when you start adding odd-orders to it- it makes the resulting waveform wider on top and bottom, without changing the frequency. I think the only way you can interpret that is 'odd orders are enhanced'. My take on it anyway...

Eldartford, I'd love to hear about your results!