Now, I'm confused.

+2 Ipretiring. Diffusers diffusers diffusers. 

+3 lpretiring
My soundstage became larger/bigger/deeper only after I installed room treatments, and I'm listening to SS, not tubes.
I'm using Primacoustic panels, purchased from Sweetwater Sound.
No vested interest, just a happy customer.
Read Jim Smith's book Get Better Sound.
Do what it says, before you spend any more $ on gear.
Your room will dictate 50% of what you hear.

I disagree that changing preamps will not give you are wider soundstage. 
Definitly try a tube preamp. I had owned Adcom 555 in the past. While they can be a very good sounding piece of equipment, they can be somewhat cold and clinical, especially pairing solid state with more solid state. I made a huge improvement in my soundstage by replacing my solid state preamp with a vintage tube unit. Namely a Dynaco Pas3. I have graduated to Audio Research tube preamps over the years, never going back to solid state preamps.
Good luck,
Mg16

How would that work? Better channel separation?

I've owned pretty much every Vandersteen speaker from the 1's all the way up to my current Quatro's.   While I agree that changing preamplifiers can definitely affect the soundstage, I don't think that it should be your first move.  As said before, Vandersteeen's are very sensitive to positioning, especially tilt.  It can make a night and day difference.  They also need space to breathe, so set up is very crucial.  It also happens to be free.  I know that the idea of buying a shiny new toy is always appealing, lord knows I've fallen victim too more times than I'd like to admit, but no matter how good your front end is, if the speakers aren't position properly, you'll never get all the potential out of your system.  Look at it this way, its something you're gonna have to do either way.  I'm spoiled because the local Vandy dealer is one of my best friends, so anytime I make any changes he comes over with his laser levels and spends the day making adjustments.  Its tedious but make a HUGE difference.