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IMO I think that amps can and do have a sonic difference - albeit very slight in some instances. I know everyone will pull out the "show me the differences in measurements" card but some things just can't be explained or heard in measurements.... Science is one thing, trial and error is another.... I just don't give a cr@p what these people say... Spend enough time with enough different equipment and you'll know yourself... I won't be one to tell you what to buy...
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They can sound different when they are not level matched for precisely the same volume (which is enough by itself to explain why so many people believe they hear differences between different amplifiers) or when one is out of spec (i.e., malfunctioning) or when one is used beyond its design capabilities (which happens more often than some people realize, as some speaker manufacturers do not properly rate their speakers for nominal impedance) or when one is driven beyond its limits (e.g., clipping).
Now, if there is a real
audible difference, people can hear it in a properly conducted double blind test reliably. There is usually a
measurable difference, even between different examples of the same model or between the right and left channels of the same amplifier, and if people had the magical abilities that they pretend to have, they would constantly be complaining that their right and left channels sound different, and that they can never get them to sound the same. This is particularly true when one considers a system in a real room, where the sounds coming from one speaker will never be precisely the same as from the other, even if they were both driven from the same channel of an amplifier (i.e., even if given an absolutely identical input signal). The fact that one rarely hears audiophiles complaining about the difference between their different channels proves absolutely that they hear far fewer differences than they typically imagine.
It is worth remembering that people often believe they hear a difference when they are told that something has been changed that they believe matters to the sound, even when nothing was changed at all. It is a very curious thing about human perception. So, without a proper test, someone believing they hear a difference is meaningless.