The $3,600 gamble

Last lemming
I would at least try Randys advice above ,
 you can get a ball park idea of timing etc.
 My take on this is consider the measurements of Vandersteens
 They are by design a unique product of form following function and considered superlative.
A quick look and a Zu product there is a forest of stored energy within the  Box design. Of course you may be taken by the fashion of the design.

Some other thoughts to consider take 3K and check out a pair of the new Belles Aria Mono Blocks.
  Run some AQ wires and interconnects to your existing Vandersteen speakers make sure you have a decent Phono stage.
 The Vandersteens in my opinion are a more sophisticated foundation to build on allowing you in the long run to achieve more solid performance.

 Dealing with your set up tilt back,Bases filled and snug, proper power response speaker position, perhaps wider spacing speakers, reading manuals
 doing the basics etc.
I have seen many come full circle with Vandersteens, chase their tail in denial for years and come back home to music at mamas.
 JohnnyR
 Vandersteen dealer
 

Johnny thanks for your response. I wasnt clear - I apologize. Inhaven't used the a Vandy's for years. My main rig is Thiel. The system the Zu's are going in is my non serious rig in the man cave. Ive been using the Zu bookshelfs there, good dynamics but no bass. I'm selling the  Vandy's to recoup my outlay of cash. 

As a Vandy and Zu owner, I can attest to the virtues of both speakers.
I also have a pair of Omen bookshelf speakers, but coupled with a pair of Hsu subs for my office system. 
My main system has the Vandy 3a sigs with 2w subs. 
I can honestly say the main system is the better of the two. But, taking intoconsideration the differences between the enclosures, I have to say the Zu Omens are pretty darned good.
So, I think you will be pretty impressed with the Def 2's. And, as you say, you do have a return policy should they not be what you want.
As always Johnny R gives good advice. Those Vandy's have a 'je ne sais quoi' that keep me in thrall. As much as I admire the Zu, a pair of Quattro's may be in my future.

I bought my ProAc Studio 148 towers without an audition, but I did audition similar ones in their reference line.