I have been listening to, designing, and evaluating the distortion of amplifiers in the lab for 40 years. Any goof who says amplifiers all have low distortion or all sound the same is an incredibly ignorant person, in my opinion.
And as for the dorks who ask if you have data, statistics, or graphs to show this or that, let me ask you this; if you have a blue car, do you need statistics or graphs to tell you that it is blue, or do you trust your eyes?
I can HEAR the difference between amplifiers that have low distortion (for example a Musical Fidelity M3i or a Bryston 3B-SST), and those that have higher distortion (for example ANY HT receiver that sells for under $1000). You can throw your sophomoric challenges at me all day long, and my EARS still hear the difference clearly, just like my eyes tell me my Corvette is blue and my Prius is gold. I DO NOT need no stinking test, survey or graph to tell good-sounding music from distorted crap music.
Furthermore, I have tested dozens of amplifiers with a Hewlett-Packard Distortion Analyzer costing $30,000 and I know exactly how much distortion they put out under real-world conditions with a speaker for a load, with its large capacitive reactances and inductive reactances, which cause the kind of cheap-ass amplifier in most HT receivers and less-expensive stereo amplifiers to distort like crazy (often 5-10 % with a speaker for a load).
The engineering, component parts, and packaging needed for EACH CHANNEL of a 100 watt amplifier that will produce less than 2% distortion playing REAL MUSIC into a REAL SPEAKER will have a retail cost of $500 and up! Realistically then, don't expect any stereo amplifier for under $1000 to sound all that great (or any HT receiver under $3000).
I spent years designing amplifiers and I can tell you that decisions on which transformer, capacitor, or transistor to use are COST decisions which determine the distortion of the final product, and there ain't no free lunch.
That is why professional audio people whose livelihood depends on their ability to know what is what when they listen willingly spend many thousands of dollars on high-quality low-distortion amplifiers like Brystion, Audio Research, Ayre, Musical Fidelity, and Mark Levinson; they know the difference in sound quality (low distortion) because they can damn well HEAR THE DIFFERENCE!!!
If YOU can't (or won't) hear the difference, just crawl under a rock and kindly don't challenge people more knowledgeable than yourself. Go away...you bother me.
P.S.- Anything I have ever heard from Bryston sounds wonderful, and when they are tested for distortion with various speakers, their distortion tests as good as they sound.
The Parasound A21 is pretty good and the A23 is a piece of shrill high-distortion crap, on the bench or in actual listening.
The Musical Fidelity M3i tests well and sounds very good.
Audio Research equipment has very low distortion and sounds wonderful.