External Crossover/Vandersteen Sub

You will gain more by getting a fixed value x over.

What is the difference between the fixed crossover vs using the wx2

The WX-2 is used to find the hi pass filer value that you think sounds best. If you have an amp with an input impedance 50K,you try the 50K setting , the numbers should be on the bottom of the box. When you find what K setting you like order that value hi pass filter from Vandersteen. The WX-2 hinders the quality of sound.

They're four options with the Vandersteen crossover:
1. Use the crossover box as you are now using (lowest fidelity)
2. Use a fixed crossover (which if you read the Vandersteen instructions is what you are suppose to use after you use the WX-2 to make a decision which one.)You purchase these from Vandersteen.
3. Make your own fixed crossover. There was plenty of discussion back about 4 years ago about this. It's not hard and gives you the option to pick the capacitors you like best. As far as I know, Vandersteen uses REL caps(yep, I took one apart!) Some other choices are Hovland, Infinicap and a higher spec Reliable caps.
4. The best option is to get the Model 5/5a crossovers (and now Quatro.) They are right expensive new but can be found from time to time for about $450-$500 used. It is battery biased and offers the most transparent sound. They are available single ended or balanced.
Whatever you do, move on from the WX2!