Do you have some measurements of Stax impulse response then?
Also, when you say "accurate reproduction of a signal" are you saying the FR should be flat? I remember reading articles on measurements and headphones that say a perfectly flat FR would sound horribly bright to the human ear. Something to do with the proximity of the driver to our ears. Then there's also loudness curves and HRTF to take into account.
I believe most Stax models have a largish soundstage due to the housings and driver size. The Lambdas (which people seem to generalize as the electrostat sound) have a cool driver housing design, where the backside is actually sloped to help with increasing soundstage much like angled drivers. The Sigma speaks for itself, and the SR Omega has massive drivers that are slightly angled which makes the soundstage large. The O2 and small monitor headphones (SRX) are generally described as having smallish soundstages. Then again, I've always took "airy" to be more related to the presence of highs rather than the size of the soundstage.