For Vandersteen 2Wq users.

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)

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.

The 2Wq is designed to operate off of the speaker outputs of the amp. This was done by Vandersteen so that the mains and sub would have the same basic tonal character in the lows. I really do not know of an amp that will not pass a low frequency amplified signal that is inputed into the amp. Most amp problems come about with interfacing with the speakers(such as BEMF from the main drivers)The signal taken from the amp does not effect the amp due to the extremely high impedance of the sub input. If you want to use low level inputs, then the V2W is the one to look at. It is designed more for home theater than music. I also agree that 2 2Wq's are a must for high definition stereo. There is bass separation and soundstage cues down into the low regions from modern sources.
I think if the amp is slow in the bass it could make for a slower sounding sub. I sorta ran into this accidently with the changing of my Theta Dreadnaught to the Belles 350A. The Theta was never known for its bass but for the mids and highs. The Belles is quicker and way more defined through the lows which I feel was reflected through the sub as a quicker, tighter sub performance. However, I do have a caveat. The Belles drives the 3A Signatures better with a tighter, more driving bass line(best I've ever heard on the 3A Sigs)So, this difference could be imparted to the sound through the mains since they are now quicker. With a 6db filter, you do get a considerable amount of frequency overlap.
Sorry, but I'm not a big fan of the REL subs. Also, I find them as difficult as the Vandersteen's to set up correctly. Overall, I do not like the way they blend or sound for that matter. I know a lot of audiophiles seem to like the REL but I find that the Vandersteen sub is easier to make disappear. I am convinced that Vandersteen's sub theory is a more accurate solution and that's what I look for in a system. I have some real questions about REL's theory on this(especially their high rate crossover)but this is me. I'm also for whatever rocks an individuals boat!