A loudness button is a crude device that is correct only at one volume level -and that is assuming it has been designed correctly which is questionable in many cases. What really is needed is loudness compensation to the correct degree across the spectrum, linked to the sound level at the listening position (i.e. initially calibrated for different volume control settings in one’s own system/room with a microphone).
Or set up for yourself using DSP
As for soundstage, the only suggestions I have are play with speaker and listening positions, including degree of “toe-in/out, and possibly room treatment.