The way to get an excellent sound stage is to make choices that musically work together. The first thing to think about is the arrangement, and what you want to convey. Should the drums be really loud in the mix? What should be leading? Should there always be a lead? What's driving the rhythm?
Once you know what it's supposed to sound like, start by putting things together, bringing things forward (louder) in the mix. Instruments that aren't driving the rhythm (like not the kick or bass) can be panned to add some space to the mix and lessen frequency conflicts. There's a mix (which I should find) of Joyful Joyful, where nothing is exactly in the centre apart from the lead vocal and it really brings the ear to focus on the voice in a way you don't often hear because of "the way things are done".
The pros will also separate out frequencies by applying subtle eq filtering and multi-band compression to avoid multiple parts fighting for the same space in the sound stage. Compression on various instruments can also make them blend in better, creating a tighter sound and a cleaner sound stage. I know this violates your desire not to change the tone of anything, but realistically you need to think about making everything sound good together, not each individual instrument on its own. Flour tastes rubbish on its own, but when you add eggs, fat, sugar and heat it can be quite nice.
Another technique to push things back in a mix is to add a small amount of reverb which has a delay of around 20 ms or so. Not enough to hear, but enough to make the ear think that the instrument is further back than before.
Ultimately you just need to experiment. Add in a technique at a time, get the most out of each technique that you can before adding in another one. You can do really nice mixes just with panning. This just takes time.
Does that make sense? Ultimately you need to experiment, in