Now, I'm confused.

In my opinion, just about all your equipment could be upgraded for a better soundstage.
The Adcom is a great amp, but if it hasn't been serviced lately, I would look into it. I think Musical Design does upgrades for Adcom and they weren't very expensive. (In fact, I just sold my 555. I found it to compare favorably to my McCormack DNA-1-which is 20 years younger).
The NAD preamp would be my first choice to upgrade, though. If you can provide a budget, maybe we can give some suggestions.
Lastly, the Dual, though a renown workhorse, could either be upgraded or at the least, have a new cartridge installed. Since I have gone to the digital side, I can't give any recommendations.
The Vandies are probably the best part of your system and will sound better with each improvement you make. 
HTH
Bob

I've setup quite a few pair of Vandersteen 1/2/3 series of speakers.  IMHO, the most crucial setup aspect is the TILT back.  This time aligns the speaker drivers to your listening position.  Get this right and set just a smidgen of toe-in and you'll get a pretty decent soundfield.

I agree with the few above that you could upgrade most of your gear.

By simply changing the fuse in my Bryston pre-amp to a SR Blue the sound stage improved; how much?  I don't really know, but it did improve and for only $150 

+1 1Graber and others.

I totally believe changing the preamp -- and perhaps the amp -- will give dramatic sonic advantages. Not familiar with Vandersteen speakers, but am sure people who commented above are right.

Oh, yes -- a cartridge change always offers huge possibilities for improvement in sound. What do you currently have?

What kind of budget do you have?

G

"I doubt changing the preamp will give you a wider soundstage"

Wrong!

The NAD's always sound too dark and confined to me. The GFA555 is capable of a very nice wide sound stage indeed. If I were the OP I would find a GFP565 this is a great sounding pre amp and they cost around $300 used. Sold new for $800 and it has a great phono section too.

Matt M