For Vandersteen 2Wq users.

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.

Let me "piggyback" on Bigtee's post. I have a stereo pair of 2Wq subs in my system (Vandy 3A Sig main speakers), and found that there was a noticeable improvement in overall sound quality when I replaced the regular ($125) crossover with the crossover units used with the Vandy Model 5. The M5 crossovers come in two boxes -- one for each channel -- and have much higher parts quality. In addition, the M5 crossovers also have an internal 9-volt battery that keeps the capacitors fully charged at all times. While the M5 crossovers are certainly not cheap ($600 for single-ended, and $800 for balanced), you can occasionally find a used pair for sale. The M5 crossovers are more transparent than the "standard issue" crossover, and also (to my ear) seem to provide a more seamless blend between the main speakers and the sub(s). Let me close by making a further pitch for a stereo pair of 2Wq's, rather than a single sub. There is a BIG performance improvement with the stereo pair.

This is interesting information that needs to be seriously considered. The Vandersteen has a great reputation, but it seems to be a bit on the demanding side to get it to sound its best. And when it comes to building or wiring anything for myself, well, I can burn water. How bad is that? My birthday is coming up soon and I have been wanting to give myself a subwoofer as a present. My choices are narrowed down to the Vandersteen and a Rel. The Rel may be the choice of someone how has less time or less experience with sound systems, but wants something musical.

Choices, choices, choices...

One more questions. Since the Vandy is wired in a parallel method, along with the mains and only samples the source, does this also mean that if my amp is not producing good low frequencies, then the Vandy won't push those frequencies, either? In other words, the Vandy can amplify a signal it doesn't get, right? Can the Vandy get hooked to the pre-amp instead of the amp?

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!