The 3x difference in price you quoted only accounts for components. When you take into account material for cabinet and your time to build, the difference becomes smaller. In fact, I would argue that because the cabinet for the Stage-4 is smaller, cabinet time and material would be less than the Towers. Compare the cost of finished speakers between Stage and Towers on Wayne's PiSpeaker site.

In terms of improvements, I believe the Pi Speakers with compression horns are in a different league than the Studio Pi's. I have not heard the Stage-4, but have built and lived with the Towers and the Theatre-3. From top to bottom, the Theatre is more refined. Both the Tower and Theatre-3 go deep in bass, but the 3 is more controlled and tight in sound. Midrange is smoother in the Theatre, without the bite in the upper-mid that the Tower has. None of this is saying that the Towers are bad. It's just that the Theatres are better. I'm certain that the Stage is even better than the Theatres.

In terms of trying to get more bass than the Towers, I'm pretty sure you will NOT find it in the Stage-4. The Stage offers improvement in quality, not quantity. If more bass is all you're after, an option would be a subwoofer based on a LAB-12, which Wayne has plans for.

Gar.