Now, I'm confused.

Speaker placement is the key. Play around with it and see if you get better results. Use a tape measure and a big dry-wall T Square to help you get exact placement. You'll need to consider the distance from the front wall (the one behind your speakers), the side walls and the listener position. You'll also need to consider speaker toe-in and tilt. Do you have the instructions that came with the Vandy's? If not, I think its on-line at the Vandersteen site. Richard is pretty specific about placement.

Once you have your speaker position optimized, you can consider electronics upgrades. But I think your wasting your money with electronics until you first get speaker placement optimized. The only caveat may be the Dual TT setup. I'm not familiar with that TT and you don't mention what cartridge you're using. Is it the only source? Do you have another source you can try? Does it produce the same soundstage issue you are hoping to resolve?

"I doubt changing the preamp will give you a wider so"
over 2000 posts and you are still giving dubious advice? yeesh. the type of preamp a person uses and whether is uses SS vs tube is a well-supported concept, with literally thousands of pages of forums delving into this concept.

The Saga would be a good start, it has tubes which is well-known for giving a either a larger soundstage, or at the least "sounding" like a larger soundstage. Let me explain.

There is a concept in audio that certain electronic devices give off different types of distortion: higher order harmonics (a type of distortion) versus lower order harmonics (distortion). the tube distortion is, to me and many others, the preferable type of distortion, b/c it doesn’t hurt your ears as much as from the SS, AND b/c it exudes a sonic "presence", meaning that tubes will often exude a wider and deeper sense of space ... IOWs = soundstage.

I’ve used tubes for 10 years, and I’ve been in your position. The Saga is good, but it only has one 6sn7 tube. Its a great tube, and it will provide an audible difference in soundstage I believe. But it is only a smaller change. I would encourage you to get a PreAmp with 2 tubes for the more significant soundstage that you are after.

for example, I have and use the Schiit Lyr 2. It is both a preamp and headphone/HP amp. it has 2 tubes (ECC88 type which will take a few different types of tubes: 6922, 6dJ8, etc). The Lyra 2 is listed under HPs on the Schiit site. The preamp section is intended for smaller desktop systems, and has a hi/lo gain. I use it will a full-sized bookshelf system with a pair of tube monoblocks I built and a pair of Zu Omens. It sounds fantastic, and I change the hi/lo gain depending on the source (LP vs DAC, etc) and the recording. modern recordings are fine on low gain. older recordings (70s) often need higher gain. (the Lyr only has 1 input, so you may want to get an external device with more inputs, Schiit makes one in fact, but only allows 2 ins, but there are very good options out there)

The Freya would be great: it has 4 6sn7 tubes and all the tube/ss/passive options. if you dont need the XLRs, dont worry, just dont use them. It is an unbelievable price for what the Freya offers.

IOWs, you can get a used Lyr2 for the price of a new Saga. And you can get a used Freya for the price of a new Lyr 2.
I would say, just go for it. you have a good supportive system and great Vandersteens! one of the best selling audiophile speakers of all time

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