This is a curious little theory:
"Carver has always enjoyed winding up the press with what are known as 'the Carver Challenges'. Three years ago, Bob took on the American high
that he could make one of his more affordable amplifiers sound just like whatever cherished high-ender his critics cared to name. The technique he employed was what he calls 'Transfer Function Matching', in which the differences between an input signal and an amplifier's output are used to identify its 'sonic fingerprint'. By inverting an amplifier's output and summing it with the output of a second amplifier, the amplifier can be tweaked to sound like the first. If the two are absolutely identical, then a null will be created by complete cancellation. Any differences will be the distortions or by-products to be removed for tweaking the second amp to sound like the first amp."
See it in context here: