Ok, since I brought up THD as a bad measurement, let's take a look at what, exactly, is being measured. A standard THD test measures harmonics generated from a

Ok, since I brought up THD as a bad measurement, let's take a look at what, exactly, is being measured. A standard THD test measures harmonics generated from a sine wave at 1khz, into a resistive output load. So, how much does a 1khz sine wave represent an actual musical signal? And how much does a resistive load represent speaker impedance? The answers are "not at all", and "not at all". Do you see the problem yet? Of course you do. And that's the problem with amplifier specs. They're measuring what is easy to measure, not what reflects sonic behavior. And it gets worse, because it leads to designing for specs rather than designing for sonics. The easiest way to get "low distortion" spec-wise in an amp is large amounts of global negative feedback. But the negative feedback loop isn't just the outgoing signal. It also picks up back EMI from the speaker. A speaker is an electric motor, but it's also an electric generator hooked to a spring. Energy put into it is stored by and distorted by the suspension, then fed back into the output of the amplifier in a time and phase delayed manner. Yuck. And unless the output of the amplifier has infinite impedance (which it does not), then that back EMI is getting injected into the global negative feedback loop, doing things an 8 ohm resistor in the THD test doesn't do to the signal. The negative feedback loop is reducing input distortion, but increasing speaker EMI. At which point the "distorted" impedance-matched output transformer of an old school tube amp starts looking like a much better proposition...