Cross-talk and distortion, chief soundstage contributors...

I am not of the opinion that the room accounts for the majority of the sound stage. It may be one component but that is only part of the equation.

Right. Soundstaging is almost entirely having the speakers precisely symmetrical and in phase. Everything else makes a difference. But none of it matters if the speakers are cockeyed and out of phase. Anyone doesn't get that flunks Audio 101. 

A more relevant amplifier characteristic is the ability to fully and believably render the complex harmonic structure of different instruments. When these are not done well then the instrument never does sound truly believable and so it is impossible for the ear/brain to ever tell you its really there. Yes the famous Fremer line, there's more there there. 

This is another one we don't have a specification or measurement for but people can hear it, they know when its there and when its better and when its worse, and the amps that do the best at this also do the best at throwing a believable sound stage.

Erik is 100% right .. it's the room.

Um, hello??

I'm focusing on the amplifier, alone... Again, the issue here are the fundamental (amplifier) qualities involved in soundstage. 

OP specifically indicated amplifier qualities involved in soundstage. Amplifier. Not room. Not speakers. Amplifier.

Erik is 100% right only if the question is focusing on the room alone. Then he would be right. Room. Amp. Two different things. If you can't tell the difference maybe read more, post less?

So anyway, amplifiers. Sound stage. We've eliminated crosstalk as a factor. What's left?

Well, not much, at least not if you're talking things that can be measured. Distortion doesn't matter. Plenty of amps with high distortion that image better than amps with measurably lower distortion. So we can throw that one out. S/N, dynamic range, hard to think of anything we can measure that would work as a reliable indicator of an amps ability to throw a palpably wide and deep stage.

That's the problem, listening99, audio always leaps in one fell swoop into areas where it becomes painfully obvious measurements are weak at best. More likely to lead you astray than where you want to be.

Amplifiers that throw a stage that is wide and deep and layered are wonderful. If that's what you want then I suggest you seek out amplifiers listeners have described as throwing a wide, deep, and layered stage.

Yes it really is that simple.

Staying within the scope of your tiny little window, amplifiers only, crosstalk isn’t hardly even relevant. This would be different of course if there were amplifiers with huge amounts of crosstalk. But that’s the thing. Crosstalk per se is only really relevant or significant when it starts to fall outside a certain range. Which nobody really knows what that is. But that’s the problem with specifications. Nobody really knows.

The thing of it is, if you want to begin to understand the full depth and nature of the misconception here, then think of this: you can get a really solid image from an amplifier with 100% crosstalk. A mono amp. The same exact signal going to both speakers will throw a perfect center image.

Now imagine the same amp only it now has a lot of separation, but still has some crosstalk, and instead of a center image we’re listening to a singer standing ten feet left of center. Everything else with our imaginary amp is absolutely perfect. Everything but the tiny little bit of crosstalk.

So what happens? Almost all the signal is telling us the singer is on the left. Only a tiny little bit is telling us she’s on the right. To the extent that tiny little bit matters it will pull her image a bit to the right. But also at the same time a little bit of the right side is leaking over into the left. Actually the same amount is trying to pull her image each way. So the result is a bit of smearing or loss of focus.

How much crosstalk before this becomes a serious issue? This is where your question is so narrowly focused it almost guaranteed a bad answer. Because phono cartridges have the worst crosstalk numbers in the world, orders of magnitude worse than amplifiers, CD players, just about anything else. Yet my cartridge with its lousy crap crosstalk somehow manages to throw a rock solid palpable wide and deep sound stage that would have you drooling and shaking your head in amazement.

How much crosstalk does my Melody amp have? I don't know. I have no idea. Absolutely none. Its probably in there somewhere. I just can't be bothered with such things. Too irrelevant. I deal with what matters. This ain't it.

Kind of a long way around but that’s how hard it is sometimes to show just how far off base a lot of these conventional wisdom ideas really are. Not your fault. Everyone parrots this stuff so much, so few bother to stop and think things through, its what makes this such a hard thing to do.