So, do all amplifiers sound the same?

No, but that's irrelevant. It's the same ears doing the listening.

Yes.

I once saw a documentry where some people were asked to eat some baked beans that hadn't had the orange food coloring added to them. They looked grey and very unappetizing.

The people who tried them while wearing a blindfold said that they tasted nice and were no different to ordanary baked beans. The people who didn't wear a blindfold and could see what they were eating said they tasted horrible and struggled to eat them because they tasted so bad.

There's also another baked beans study where people consistently preferred the beans that came out of a Heinz tin instead of a value tin, even where they were the same or had in fact been swapped. Also the wine test where experts described the red wine taste of white wine that had been coloured red. 

Does that not show that "colour" effects taste, rather than being unsighted.

What I'm saying is that IME there are differences. I think difference in amp signature might be specifically created to suit certain tastes.

I imagine that also speaker load and how well the amp copes with this can affect the sound.

The same amps in active setup might have less of a signature sound, who knows?

Just my opinion, YMMV.

No, it shows that your brain fills in when it expects something to taste (or sound) a certain way.

This may be partially true, but it's too simplistic (imo). Colour and taste are inextricably bound together.....probably going back to our ancestors, where being hardwired to avoid eating things of certain colours could save your life.

Using the right food colouring to make stuff look more appetizing, is very much based on known science.

And I'm pretty certain the purveyours of aircraft-grade steel-cased amps at aircraft-grade prices know something about the equivalent, inextricably linked science when it comes to sound

By the way, I do think amps sound different. I'm certain of it. But I do wonder about dm's interesting concept of them sounding the same with tone controls...on that I think it's a very big maybe.

Then again, there are those that buck the trend, and put SQ above all else.....and if it looks good as well, it's icing...

This rather ignores the one where they look the same but people's taste is dictated by the can they come out of. Very like hifi!

That's the marketing / branding element.....and goes to show that almost eveything is more complicated than it first appears to be.

Ah good, at least you accept that when someone says 'there's definitely a difference because I heard it' we can't take that at face value and it is 'more complicated than it first appears to be'.

Likewise, when someone who hasn't heard it, says there isn't a difference. 

All beans sound the same.

No they don't!