I don’t know about soundstage but lack of bass at low volumes compared to normal or high volumes is a natural function of human hearing.
That’s what the infamous old “loudness” function in cheap 70s and 80s amps was intended to be used for. Trouble is it was very crude, on or off, so other than at one loudness level too much or too little, and often misused. As I have expounded before, there would be great merit in having a sophisticated loudness control using DSP boosting bass and, though to a lesser extent, upper end of treble (or cutting the middle range) at an appropriate rate proportional to sound level at the listening position. It can of course be done manually but then you need to change settings when playing louder.
This chart shows human hearing equal loudness curves, illustrating how the curves vary with sound intensity: bass is relatively quieter in relation to mid frequencies if the net sound level is lower.
Soundstage may be due to room effects, related to speaker and listener positions and room reflectivity. You can play with positioning, also the degree of ‘toe in’ can be a significant factor.