Why do tube amps generally have larger sound stages? maybe its a harmonic effect some amp do them well some don't.  Is it distortion effects, even/odd order effects?

clarity maybe, but I've heard some older vintage tube gear throw a huge sound stage with out great detail and clarity levels. so really doesn't hold up in my experience. 

Even though the OP used a YouTube video as an example its still a valid question that many of us have experienced when trying new gear. 

Why do some amplifiers throw a bigger soundstage than others?

@rrm Two things: lack of phase shift and aural effects having to do with the 2nd and 3rd harmonics, the latter of which can make an amp seem more detailed and spacious.

I like tube amps, particularly low-powered ones, and they do seem to present a larger soundstage than many good solid state amps.  My personal suspicion is that they do so because of some kind of phase anomaly.  I don't care if it is a kind of distortion or defect, if that is the case, it sounds good to me.  I know that transformers don't behave perfectly, particularly when it comes to phase shifting, and the amps I particularly like have a LOT of iron in them--input transformers, interstage transformers, and output transformers. 

I have very little experience of tube amps, but I do remember a wide soundstage.

However, the review I watched compared two solid state amps, both class A.

still don’t understand why one would would throw a wider soundstage than the other.

Listening is a skill. It requires lots of practice and study to learn the vocabulary of standard descriptive terms. For experienced amateurs and professionals assessing sound qualities are relatively easy and comparable across listeners. Kind of like reading X-rays. For the professional… he’ll just rattle off what this is and what that is while to the untrained eye, all you see is fuzzy gray areas.

So all this discussion about how subjective sound is, or it is some trick of the mind comes from inexperience. I can take three different experiencesed audiophiles and get exactly the same description of sound qualities of a system or changes in that system exactly and consistently. Most of the subjectivity involves words like good or better… value statements about your values… what you like and don’t like.