There is apparently some relationship between frequency response and soundstage perception so you might just have to choose whatever sounds best overall. Presumably, there are EQ adjustments that would increase the soundstage.
The recording/production will play a role too. That's probably the most important thing. "Headphone" or binaural recordings should be more realistic with headphones.
There are some headphone amps with "crossfeed" which blends left & right. This should make the soundstage more realistic, since when you listen to speakers (or a live performance) the soundwaves blend in the air and reach both ears. But I assume it also narrows the soundstage.
There was Dolby Headphone and now Atmos for Headphones but I don't know if they are supposed to work with regular stereo sources (probably). I tried Dolby Headphone once with a surround track because people were claiming to get surround sound. (Dolby doesn't make that claim). It did "something" to the sound but I wasn't getting the illusion of the rear channels coming from behind.
FYI - Most people don't get a realistic soundstage illusion with headphones. The most common perception is sound coming from inside the head!
Headphone Soundstage Survey