The decision to add another 2W versus a 2Wq is a personal judgement call, as you may not notice a significant performance difference in stepping up to a pair of 2Wq's. About a year ago, I had a chance to talk personally with Richard Vandersteen while he was in Seattle to conduct a seminar at a local dealership, and I asked him about the differences between the two subs. He commented that there are two differences:
1. The most obvious difference is the ability to "tune" the Q on the 2Wq. I have found in my own system (which has a pair of 2Wq's) that adjusting the Q does make a useful, although not dramatic, improvement in the overall quality of the bass. I think that the amount of improvement will be dependent on the main speakers you have, the acoustic properties of your listening room, and where the subs are placed in the room.
2. The built-in amplifier in the 2Wq is of better quality than the one in the 2W, and if you are using the sub(s) with high-quality main speakers, the 2Wq will provide somewhat better bass reproduction.
Personally, I would not match your 2W with a 2Wq. Either get a pair of 2Wq's, or add a second 2W, and thus avoid the possibility of different audio qualities from having two different subs. The one distinct advantage to having a pair of 2Wq's is that you can tune the contour of the "Q" for use with both music and home theater.
Your post does not mention what type of main speakers you are using, so bear in mind that Vandersteen's subs are optimized for use with main speakers that have flat response down to about 40 Hz. This allows the main speakers and the sub(s) to overlap by one octave, which is a major factor in getting the seamless blend that Vandy subs are known for.
