Thinking of adding a subwoofer for more bass.

You may want to check out the Velodyne SMS-1 subwoofer controller - for use with any sub you choose. It allows room analysis below 200hz and Parametric EQ on 6 bands between 20hz and 120hz. I recently bought one for use with my Verity Parsifal Encores (and I also bought subs for use with other speakers that I rotate into my system).

This unit requires real effort - PEQ turns out to be more trial and error than I thought - but the results are really dramatic. The cost is app $600 (AudioAdvisor.com) plus whichever subs you choose.

Good Luck

Marty

Check out this thread: Add a subwoofer to Von Schweikert 4Srs?
I ended up adding a sub, an REL Storm III, and couldn't be happier.

Don't confuse more bass with lower bass frequency. A good Sub shouldn't ADD bass to the sound, only reproduces the lower frequencies your speakers can't handle (in your case anything below 37 Hz, which is not a lot and hardly audible. You feel it rather than hear it).
A lot of cd's these days just don't have a lot of bass in them and sound lean. That has nothing to do with your system and if you want more bass, than it will def. reduce the microdetail of your bass responce in case of a sub that's been adjusted to ADD bass. If you adjust the sub correctly it will not add, but just reproduce the frequencies below your speaker's bounderies. It could very well be the case that your speakers roll off at 40 or 50 Hz or something. Most speakers don't go nearly as low as told, in which case a good sub could be a solution.
But don't expect your lean cd's to sound full of bass all of a sudden. It won't happen.

Hope this helps.

To clarify my post: Hulskof's post is correct in theory, but not necessarily in practice. If your system is actually producing flat response to 37hz, you won't hear much difference with a sub. You will feel the extended bass, as he indicates.

In practice, you are almost certainly getting bumps and nulls in your bass below about 125hz - prior to adding the sub and eq, I was seeing "hills" of 11 to 12 db (reference to 80 db) and "valleys" up to 7 or 8 hz. A sub with equalization (like the Velodyne) can address this issue and you will most definitely hear a difference. I eventually acheived +/- 3.5db from 25hz to 200hz using the SMS-1, but it took a lot of time.

Note: The subs with auto EQ don't remotely acheive the same result as using the SMS analyzer to tweak and optimize response by hand. It's funny in that similar looking total deviation from flat response may sound quite different one graph to the next, but less total deviation (especially near the x-over point) definitely sounds much better (to me). In other words keep tweaking for flatter response and I'm confident you'll hear a difference.

Good Luck

Marty

The reason why I asked the question is because I was worried that the output frequencey of the amplifier was the problem because the Vandersteens go down to about 37hz.

Is this what you measured in YOUR room, or is this the spec that vandersteen publishes? If 37Hz is the response you get in your room, then I doubt adding a subwoofer will give you much other than bass bloat. What you are probably looking for in that case is bass slam/dynamics which is something else altogether.