This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Some say that they do. Some say that they don't. Some
demonstrated that many amplifier differences can be traced to
very slight frequency response difference. Let your own ears
guide you. If you want to compare amplifiers, you can do it
best in a controlled environment, such as your home, with your
music and your speakers. Also be very careful to match levels
precisely. All you need to match levels of amplifiers is a high
input-impedance digital voltmeter set to AC volts and a test
recording or signal generator. For best accuracy, set levels
with the speakers wired to the amplifier.