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.
Thanks @atmasphere
The perception of a larger soundstage can be influenced by various factors, including the design and characteristics of the amplifier itself. Here are a few reasons why some amplifiers may be perceived to have a bigger soundstage:
1. Power and dynamics
2. Low distortion
3. Channel separation
4. Transient response
5. Room acoustics
Thanks @mapman
Isn’t it nice to get straightforward and technical answers rather than responses that it’s user-error or placebo effect? Lazy thinking which blames listeners first is so enervating.