Allow me to think out loud so to speak...
Lower frequencies have longer sound waves and higher frequencies have shorter sound waves. This is why the location of tweeters is critical to the listening experience and subwoofers can be strategically places in the room. It seems reasonable to me that there must be frequencies somewhere in the mid range where the there's a shift away from directionality. Maybe an amplifiers (or any pieces of gear) ability to produce these frequencies is what impacts the sound stage. The relative timing from the two speakers is what creates the stereo sound/soundstage. I'm thinking that any degradation in these frequencies could negatively affect the timing leading to a more directional sound which would be expected to shrink the perceived soundstage.