Amplifier for Vandersteen 5A' s ?

Quicksilver V4, Atma-Sphere MA-1 mkII.3, Theta Enterprise...
Those are the only mono's in that price range that pop into my head right now.

Will

Should you purchase the speakers, why not first try them with the amp you have? That way as you audition other amps you have a baseline as a comparison.
I did that when I moved up from the 3A sigs. to the 5As. I kept the Dreadnaught amp that I had been using while trying others. I reluctantly let go of the SS Dreadnaught after using a ARC tube amp in my system. I really wanted the Dreadnaught to better the tubed amp for two reasons. One, I had bought into the idea that SS was better when it came to amps and didn't want to admit that I was wrong and, two, I didn't want to spend the money.
I'm sure that there are many amps that work well with the 5A's but, I know for sure that Audio Research amps match very well with them, assuming that they also match well with your pre amp.

I have the 5A's and I think the best bang for the buck that match that speaker perfect is the amp that Richard vandersteen and I use the Quicksilver v4. I had rca switched to balanced and think it is woth the upgrade. They are very low maintance take a varity of tubes and sound grate. Also 120 Whatts...

I have also herd the Cary on them the 308 siginature edition and think they sound a bit more open but they are of course twice the money.

You can add used VTL sig450 to the list.

I have 5As and have been looking around too. Unless you are in hurry your Pass W-600s should work well.

I called Richard Vandersteen a while ago. After he realized I was serious, I told him that I had Pass Aleph 2s and that I was considering Theta monos. To my surprise he called that largely a latteral move (though he said the Thetas were really good) and that I should consider tubes. He thought tubes had the advantage of better image body (I forget the exact term used) over the best solid state.

Our conversation was cut short so I hate to say too much with out a more extensive talk.

But Vandersteens are tube friendly designs if you want to deal with tubes.

FWIW I have on my list at this time Quicksilvers (max value), VTL sig450 (used), ARC VTM200 (use, more $) and if I really go wild ARC ref 210s or VAC Phi 220s(if I can find a deal). If I stay solid state the Theta monos. Still looking, got a few months yet.