I have to disagree with the idea of mostly high frequencies being associated with a taller soundstage. I just visited a friend with a wonderful system and we listened to some organ music. From the lowest pedal notes to the highest keys, you could "see" the pipes going up the front wall (9 feet tall) to the ceiling where all the music emerged near or at the wall/ceiling interface about 8ft behind the speakers, with no sound audible from the speakers. Glorious.
It does require speakers (3-4 way) that have matched dispersion and phase at crossover, low diffraction, excellent time alignment, and ideally a rear firing tweeter. The sound should be essentially omnidirectional with the speakers sounding the same from either the front or the rear. As an example, you're not going to get a realistic soundstage with height from a pair of Altec 604s in 8 cubic foot boxes. That's not what they do. Then there's the room. Most don't have available space for a rule of thirds setup, but it's important as are traditional absorption and diffraction treatments.
All that said, the image height could still be stuck at or slightly above the speakers. If you're listening digitally there's a lot of things that will effect your soundstage. USB cables, server and dac selection, switches (I didn't believe it until I heard it), linear power supplies, storing music on a NAS, not a PC, etc. There's not a simple do "this" and get a huge soundstage secret, unfortunately. Work to get sound from being stuck on your speakers as a first step. If you listen to a live recording in an acoustic space and hear music coming from the speakers, try to eliminate it. In the Altec 604 example, you wouldn't be able to, but if your speakers meet the criteria mentioned you should be able to make them disappear.
And still, some speakers/systems will not be able to do it. Most listeners and many manufacturers don't listen for this or it's not important to them. Many are happy with a good center image with some depth. But when you hear an absolutely transparent system, there's nothing like it.