Man, Ive spent the last few years chasing this and am finally satisfied. Soundstage has so many different aspects and I don't think it's a simple answer. It's going to depend on your room and setup.
1.speakers. Your other components will have a role but nothing on a system defines how it sounds more than the speakers. Id they can clearly and truly reproduce the recording that's a huge first step.
2. Placement. Where your speakers are in the room and how they are tied in. This is super speaker and room specific. For me, I use no toe in as my speakers still create a perfect center image with no toe in. Having no toenin maximizes the width of the soundstage. I have tried toe in before I changed some other components to try and achieve more image focus but always missed the depth too much. I'll say that this part, setup, is one of the most viral things you can do. Read the directions that come with your speakers. Call the manufacturer and ask for tips. It will take a lot of time but you'll get a lot of mileage here.
3. Source. I'm an analog guy so having a turntable, arm and cartridge that can accurately pick up the recording is crucial. Without that, you'll never get proper center image or depth or width of soundstage.
4. Amplifiers. This one surprised me. Id been looking to achieve more depth of soundstage and had changed all other components, largely to no avail. When I upgraded my amplifiers (Atma-Sphere MA-1s) I was blown away by the increase in clarity, space around the different instruments and voices. This space and clarity really enhanced imaging and depth.
5. Preamp. This is the one I changed the most chasing the depth. What I found was that while the preamp is the heart of the system in a o lot of ways, it didn't have that much impact. It does define the overall tone of the system but it wasn't until I proved my amps that I was able to hear the full potential of my system.
For full details see virtual system