Thinking of adding a subwoofer for more bass.

I have the ta-30 (predecessor to the a50) and the bass information is definitely there. I dont know too much about the vandersteen 1's but try playing some bass test tones from around 40Hz on up and see what the response is in your room. My 6" driver monitors go down to about 45-50Hz without a sub so your vandersteens should go at least that low.

There will be many other factors at play such as your room size, listening position, speaker position, etc. Walk around your room to see if bass energy is higher at other points. If it is, your listening position is probably located in a null.

Short answer: since a subwoofer is a speaker for outputting bass, sure adding a sub will definitely give you more bass.

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.