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

There is no substitute for Signal to (Noise + Distortion) Ratio. But analyzing the various contributions to Noise and Distortion is not straightforward, the total Noise and total Distortion represent the sum of the various contributors. No one source of Noise or Distortion holds sway over the other sources. How many sources of Noise and Distortion are there? And what are their percentage of contribution? Hey, that could be a pop quiz!

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.

If we ignore the "chemistry" of sound, and focus on 1 component alone, then yes kind of sort of.

Truthfully you don't need perfect crosstalk. Look at how poor LPs are compared to CD's and they can image wonderfully.  I think about -60 dB in the midrange is a great number. 

Same for distortion, lots of tube amps have relatively high distortion and image great. :)

So, yes, you can built a crappy amp with high cross talk and so much distortion you can't listen to it, but that only goes so far.

But in today's market, with many good amps measuring well in both dimensions, the room and speaker integration are FAR more important areas of focus to me.

Best,
E

And, don't forget about harmonic distortion.-Especially odd and even ordered harmonics, which the human ear can distinguish and play a role in how the sound of the amplifier is perceived.
Bob