Why do some amplifiers throw a bigger soundstage than others?

What you heard is a subjective opinion by a person who has not done a careful blind test. Therefore the premise that one amp has a wider soundstage is suspect.

IMHO, clarity is one of the most significant contributors to soundstage. If your speakers have good phase coherency, and are mated with an amp with good clarity, you should get a good soundstage if properly setup.

@jasonbourne71 

Right on.

@jasonbourne71 +2

Mike

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.