soundstage *depth* is a whole 'nother issue - but basically similar tings are happening: ya wanna optimize the speaker placement w/the wall behind 'em, usually it's a compromise w/floor-standers & monitors on stands: further from the back-wall gives better image-depth (& worse saf), closer placement increases bass-response at the expense of image-depth (& saf improves!). ya also wanna sit far-away from the rear wall. of course, small rooms are aided w/strategic sound absorbtion & diffusionn when you or the speakers get closer to the walls than ya'd like. if ya got the space for it, subs can help in speaker placement for optimum sound, imho, cuz ewe can place the monitors for optimum-depth while placing the subs for optimum low-end response. i believe this can improve the *monitors*, even if they're full-range floorstanders - i had my thiel 3.5's set-up this way, before i got my present small merets, & they never sounded better - that 10" woofer was better higher up the frequency range when it dint have to go down to 20hz, and the marchand x-over is more transparent than the thiels' equalizer, that i was able to ditch. it's amazing how big speakers like thiels can disappear when out in the middle of a room! :>)
for optimum image height, go for a carpeted floor between ewe & the speakers, nothing else between yew & the speakers, & as high a ceiling as possible, or sound treatment on the ceiling between ewe & the speakers. kinda extreme... :>)