I have a hard time understanding (all specifications being equal) how an amplifier can sound different.
First let me say that I second and applaud Bryons admonition to others who may respond.
Here are some reasons that come to mind as to why similarly specd amplifiers may sound different. There are undoubtedly a great many others, both known and generally understood, and unknown/unexplainable:
1)Amplifier specs are generally based on the assumption that the speaker has a resistive 8 ohm or 4 ohm impedance. Real-world speakers have significant reactive (inductive and capacitive) impedance components, which vary widely among different speakers, and, for a given speaker, vary widely with frequency. Loads having significant reactive impedance components are more difficult to drive than purely resistive loads, and different amplifiers will have differing degrees of success doing so.
2)Harmonic distortion created by an amplifier is commonly specd in the form of TOTAL harmonic distortion (THD). However, the audible significance of THD is highly dependent on WHICH harmonics are present in the distortion spectrum, and to what degree.
3)Different amplifier designs use differing amounts of feedback. Feedback can reduce THD, improve linearity, and lower output impedance. However, it can introduce or worsen Transient Intermodulation Distortion (TIM), which is not generally specified and which subjectively can be more objectionable than the effects that are helped by feedback. A high quality design will need little or no feedback to minimize those effects, and will therefore have minimal TIM, but will tend to be more expensive.
4)Amplifier output impedance (equal to damping factor divided into 8 ohms) is a parameter that is fundamental to the interaction of amplifier and speaker. Although damping factor is usually specified, the specs for different amplifiers vary widely, and precise prediction of what combination of output impedance and speaker will be most synergistic is generally not possible.
5)Different amplifier classes
are subject to differing kinds of distortion.
6)Amplifier power supplies generate high frequency noise, due to an effect known as rectifier commutation, which, depending on its frequency components and amplitude, can radiate or couple into circuit points in other parts of the system, with unpredictable results.
7)Ultrasonic or rf spurii that may be fed into or picked up by the amplifier can intermodulate with the audio signal, resulting in effects within the audible spectrum. The degree to which that happens will be dependent on the bandwidth, the circuit topology, the physical layout, and other characteristics of the amplifier.
8)Unspecified stray capacitances in the amplifiers power transformer will affect sensitivity of the system to ground loop-related hum and buzz (subjectively affecting background blackness).
9)The behavior of different amplifiers when over-driven or clipped will vary significantly, particularly when comparing tube and solid state designs.
10)Amplifiers have differing amounts of dynamic headroom, meaning the ability to deliver greater amounts of power than their continuous power rating, for limited amounts of time. Dynamic headroom may or may not be specified, but even if it is, amplifiers with similar amounts of specified headroom will vary widely in terms of how long that increased power level can be supplied for.
11)Parts quality will affect sonics in ways that are not readily specifiable or predictable.
As I said, there are many more known and unknown reasons for sonic differences between similarly specified amplifiers, but hopefully these will give you a general flavor.