Not many people realize that there can be a tradeoff relationship between soundstage width and imaging precision.
Strong early sidewall reflections expand the apparent width of the soundstage but they do so at the expense of imaging precision, and this can include soundstage depth. Many people prefer this expanded soundstage width, but for the most 3D imaging from a given pair of speakers, strong early sidewall reflections should be avoided.
Note also that the ear is getting conflicting messages about the soundspace you are in: It is getting the spatial cues on the recording about the actual (and/or engineered) soundscape; and it is getting cues about the playback room. We want to encourage the former but suppress the latter. The early reflections are telling us that we are in a small room, so again we want to minimize them. In particular the early reflection off the wall behind the speakers tends to impose its signature on our impression of soundstage depth. Likewise a strong early-onset reflection from the wall behind us is conveying small-room signature. Imo set-up geometry, reflection management, and diffusion are generally preferable to absorption for suppressing early reflections because absorption removes energy that would have been beneficial as later reflections, and absorption is most effective at high frequencies therefore it correspondingly alters the spectral balance of the reverberant sound (which is usually undesirable).
There is of course a lot more to 3D imaging than this.
Duke
