Which speakers have the biggest soundstage

I agree on the tube part. Look at electrostats or hybrids with Electrostats. You cant beat them for soundstage open-ness:
http://www.cadenceaudio.com/speakers.html

Ditto on the Harbeth's. For me, the HL-P3 series, old or new. On the vintage front, especially if you are going to use tubes, the Spica TC-50 is a favorite, and throws a huge soundstage.

i've run Coincident Partial Eclipse II's w/a sli-80 for over 5 yrs now w/out any desire to upgrade...good luck

Soundstage is a function of the acoustical interaction between the speakers' dispersion, room reflections and placement.

I have found that dipole or omnidirectional speakers provide the biggest soundstage. Front wall to speaker distance affects the depth—minimum 3 feet, preferably around 4-5 feet. Pulling your speakers out from the front wall also increases the ratio of direct to reflected sound, a secondary benefit.

Ojgalli, Speakers, dispersion, room reflections and placement mean nothing without the proper electronics.

Amplifiers, preamps and source components play a huge role in the presentation.