Amplifier specs, does they matter?

Dasign, I would stay away from amplifier that exactly doubles power because in order for this to happen power supply has to be strong enough while output has to be "tightly regulated" which is another word for the "deep negative feedback". In addition many new amplifiers have small amount of power supply caps and tiny transformers since they use ultra quiet line/load regulated SMPS.

Psaq, In normal test for THD amplifier is fed with pure sinewave ant tested at the output for anything else present. Not only that this test says nothing about THD when driving your speaker but also won't show any high order harmonics manufactured by deep negative feedback in Transient Intermodulation.

40dB of negative feedback means that amp was designed with 100x higher gain that is reduced back to normal by feeding output signal to the input in opposite phase (cancelling 99% of the input signal). This scheme is wonderful and improves everything 100 fold, but when rapidly changing signal (music) is applied amplifier for a moment has higher gain (because of input/output delay) and output overshoots (signal fed back to the input is too late). It can be easily shown when testing amplifier with square wave. This overshoot translates into unpleasant high order odd harmonics. Such amp will have fantastic THD, IMD, DF etc. specifications but it will sound horrible - guaranteed. Even power ratings can be very misleading. Because of all that I think that specifications are pretty much garbage. I would use them only, as Al stated, to exclude certain amps that, for instance, cannot drive my speakers etc.

Mapman, there is nothing wrong with use of negative feedback but it has to be done right. Using deep global NGF to achieve better spects at the expense of the sound is what I'm against. Why would anybody need DF=1000 if choke in series with the woofer limits it to less than 100 anyway. Speaker impedance, mostly resistive, already poses limitation. I can understand output impedance 10x lower than 4ohm to provide good damping (DF=20@8ohm) but above it it is just nonsense. Class D amplifiers have very high DF at low frequencies but it wasn't intended IMHO. It comes from the fact that speaker is always shorted between two low impedance points (GND, VCC) by very low impedance Mosfets switches. Some negative feedback, used to linearize output, reduced output impedance even more.

Extending frequency response to >100kHz might be a good thing to avoid phase shift in audio range (poor summing of harmonics) while reducing THD to fraction of percent should be enough. Good design involves quality components combined with very linear topology. Frequency limited input circuit should be followed by extremely fast output stages.

In short it should be very linear and fast to start with instead of fixing it with NGF.

It should be just enough of negative feedback to reduce THD, IMD to fraction of percent (only few times reduction). Bandwidth will increase but you need to reduce it at the input back to one that amp had before feedback was applied. This will prevent TIM completely but amplifier has to be fast to start with.