My take on this after many years of listening and deliberation is to expand the thread title into:
Most amplifiers sound more or less the same, most of the time.
What do I mean by most amplifiers?
Well, the typical SS stuff churned out by the likes of Audiolab, Cambridge, Cyrus, Pioneer, Technics, Quad, modern Naim, it's a long list.
They all measure well in all key area and lack significant quirks.
Most loudspeakers these days are fairly benign loads though tend to to sit in the 4-6 Ohm region these day. Power is cheap.
So you take the typical integrated amp and the usual boxes from Mission, MA, Elac, etc and the results are boringly predictable.
Sonic differences between amplifiers on line sources will usually be inaudible, especially under the conditions in which most of us evaluate and compare.
But that's not the whole story of course, and to horribly paraphrase the late Peter Walker - i
t's the interface, stupid!
A couple of examples.
Tube pre-amp measures great in nearly all areas, low noise, low distortion, flat reponse. Has a 1.5K Ohm output impedance.
I go for a demo and connect it to a power amp sitting alongside with a 50K Ohm input impedance via a 1m low capacitance cable.
Sounds great, love it. I try it with a completely different but also excellent power amp, this one has 30K Ohm input impedance. Again sounds great and much like the first.
This must be a good pre-amp and I've demonstrated it can sound excellent through two different power amps. I buy it and take it home.
Connect it via 2x7m interconnects to my expensive Class A monoblocks, each with input impedance of 5K Ohms.
Now it sounds weak, warm, rolled off and flat. Yuck!
Loading is all wrong.
It's the interface, stupid.
Now I'm listening to a couple of integrated amplifiers via the phono input.
Both highly competent designs, the only significant different being that one has a steep high pass 'warp' filter attenuating signals on the phono input <20Hz.
I'm using a WTA turntable with Ortofon MM. I'm struggling to hear a different of any significance. Both sound like really good amplifiers.
Now I swap the turntable and use a Rega turntable with a Grado cartridge. Suddenly it's night and day different. One amp, the one without the warp filter sounds really poor......
The Grado in the Rega is compliance/mass mismatched just a little, and the combination is relatively undamped, sending significant <20Hz signal into the amplifiers.
One amplifier is passing this merrily into my little ported bookshelf 'speakers and the bass drivers are dancing accordingly, modulating and distorting the wanted signal along the way, the other amplifier is not.
One amplifier is now seeing significantly more power demand thanks to all of that subsonic crud, the other is not.
It's the interface, stupid.
You thought this was easy?