Thanks, Martin - and you are spot on with the room and the mixing engineer. If the mixing engineer has done a good job then I think the secret sauce for speakers that image well and produce a big soundstage is polar response (ie driver radiation pattern or directivity). The shape of the upper baffle on the LX521 is designed to ensure that the radiation pattern is the same (well, as near as possible) for all frequencies even though there are three different drivers involved. Siegfried Linkwitz, Earl Geddes and Floyd Toole, all designers that I admire, are proponents of constant directivity design for speakers.
Some of the best new speakers I've heard in the last few years are the Kii Threes and the Dutch & Dutch 8Cs. Both are using DSP to control directivity and I've always been impressed by the soundstage and imaging whenever I've heard these speakers. I guess you can say they are a new generation of speakers in a way as they are almost completely self-contained - fully active with amps for each driver in the box, dsp and connectivity for just about whatever source components you want to use. But mix and match they are not so may take the fun out of this hobby for some.
When it comes to soundstage I think the physical size can have a major impact. I've just started experimenting with my Maggies in a 5.1 configuration (MG2.5Rs for L/R, CC2 for centre, MG1s for surrounds) together with the LX521 bass modules. This is as close to what Toole calls envelopment as I've heard. The images are more diffuse than what I get with the LX521, but are positioned nicely in the soundstage. Although the LX521 can throw a big soundtage the Maggie 5.1 system is grander in scale and has a very relaxed, but still detailed presentation. I've never had much desire to go down the multi speaker route, but they have been quite a revelation.