Good amplifiers should not and do not have a sound of their own: they should be what Peter Walker once called 'a straight wire with gain'. However, this is conditional on three things, apart from basic design flaws. First, they should have enough overload margin on their inputs. Second their frequency response should not be load dependent, and third they should have enough power to deal with dynamics peaks without clipping. Get it wrong on any of these, and amplifiers will have a sonic signature. Fortunately, it is easy (and cheap) to get it right, even if some (or even quite a few) expensive audiophile amplifiers fail the test. If you are really keen to compare two amplifiers, you have to do so with levels carefully matched, i.e within 0.2 dB. You cannot do that by ear or with a sound meter - you will need a Volt meter. Get this wrong, and the louder one will always sound better.
For authorative discussion, see the site of Douglas Self, one of the world's most respected amplifier designers: http://www.douglas-self.com/ampins/pseudo/subjectv.htm