It depends on what you mean by "sounds the same".
If I play the Beatles and hear the Beatles and it actually sounds like the Beatles on 5 different amps ... do they sound the same? Maybe ... maybe not.
All amps sound similar in that you hear what you play, but I know for a fact that Amps do not sound the same ... but again they sound similar. My Yamaha and Rotel sound very much a like. However, my Onkyo do not sound the same as the other two. (From Memory) The Onkyo have a Damp Factor of about 50, where as the Yamaha and Rotel have a Damp Factor of 250 or more.
Now keep in mind that a somewhat artificial Damp Factor can be created with negative feedback. All amps use feedback to some degree to stables the amps. It is just a matter of how much and by what means.
Whether that explains the entire difference between Onkyo and Yamaha/Rotel, I can't be sure. The side-by-side the difference is very clear.
Now, does that mean one amp is bad and the others are good? No. They all sound good ... just slightly different.
Next it depends on the class of amps, and I mean that in two context - Class-A/Class-AB/Class-D/an assortment of other hybrid classes, then price class. I don't expect a £150 Receiver to stand up to a £15,000 Amp. Though admittedly a £15,000 amp is a bit on the ridiculous side.
But this is a moot question, the question isn't does it sound the same, but rather do you like how it sounds? Does it please you? If it does, then - that's a good amp regardless of price.
Last, is the element of sanity. To me it is insane and pointless to pay anything more than about £5000 for an amp, but you are free to do so - it is after all your money. And in reality, even £5000 is a bit over the top.
Again, it doesn't matter of they all sound the same or if they all sound different, and long as you can fine one you like the sound of.
There are friends and enemies in both camps. Those who absolutely swear that all amps sound the same, yet oddly they do not have a £150 Stereo Receiver. I had one 'all the same' guy who eventually admitted, after a great deal of pressing, he actually had a NAD Master-Series amp. These are expensive. His justification was that he got a deal and it had features he needed, but that particular amp was not really long features, it was pretty basic. So to me - he was full of crap.
Another camp is all the same are those who have cheap amps, and what they are really saying is that they currently have the best they can afford and are a bit self-conscious about it.
Lastly the camp that says amps are different and therefore sound different, but not grossly so. But it doesn't matter as long as you have an amp that sounds good to you.
It is something of a pointless argument or discuss because like cable and a range of other hot-button topics, it never can and never will be resolved.
Steve/bluewizard