Which speakers have the biggest soundstage

Harbeth SHL5

I agree with the two above. I slightly prefer the sound of the SHL5s and they dont need to be in the corners to get there best bass.

I suggest looking into used Merlin VSMs.

The speakers that are set up correctly and use both tube preamps and tubed power amps for electronics. I found the tubed power amps have by far the biggest effect on soundstage then the input tube on your pre. I know some will claim otherwise but if you try it you will see that it is the case.

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