For Vandersteen 2Wq users.

Do you have 2Wq's? I'm really curious about these subs...I've read some really glowing reviews.

The crossover (or filter) is designed to roll the bass out of the mains at 6 db per octave. This is phase correct. In this respect, it does change the sound in a highly controlled fashion. The crossovers hinge point is set to be at about 80hz. Using good quality caps in the filter does not alter the mids and highs going into the mains. The idea is to take the workload off of the mains and pass it to the subs. It allows the use of a lower powered main amp(if desired) and, if anything, cleans up the midrange and highs. Dynamics are improved along with clarity.
The Vandersteen 2Wq's are designed with what Vandersteen calls a "Phase forward error correction." All this does is restore the bass via the subs in exactly the same amount as the filter has rolled it off out of the mains. It extends the bottom to about 18hz through the subs. The idea is to create an extended bottom that is not lean nor bloated but ACCURATE. Most people boost their subs up too much. If you hear the sub, its too high in level. All of the sound should just exist and not be pinpointed in the space as coming from drivers(speakers or sub[s]. If you like the effects of a loud sub, so be it. An inexpensive Radio Shack SPL is a good source to measure your actual frequency response.
You can read up on all of this material on the Vandersteen website (www.vandersteen.com)

What he said :).

I would like to add that from my lengthy experience...The passive x-2 filters will change the sound of yours highs and mids in a negative way unfortunately. The active unit is the only way to go for transparency sakes...it is expensive but to me the vandy sub is almost worthless with the x-2 unless you have an extremely unrevealing system. If the active is too expensive then I would HIGHLY recommend to have a custom filter built with much better caps i.e. DYNAMICAPS,Auricaps,Cardas caps etc. I had one built with DYNAMICAPS and I can hardly hear the influence of the added connection...It sounds MUCH MORE natural and open against the X-2 and is very close the the performance of the active units at a fraction of the cost. Just trying to help from my experiences..good luck!

I totally agree with Aniwolfe. You must either make your filters out of a much higher grade cap or use the expensive Vandersteen model 5 crossover. The x-2 doesn't alter the frequency so much as it just isn't as transparent as it should be. It uses a relative low grade Rel cap. I have used Dynamicaps and also Auricaps. I found the Auricaps to be more to my liking but it is a individual thing. The filters are extremely easy to make. I probably have no fewer than 10 pair laying around.
With the Auricaps, I hear no influence to the mids and highs. I felt the Dynamicaps imparted a touch of brightness.