Thinking of adding a subwoofer for more bass.

Just add the sub. It should do it for you. Don't know the Cayin.

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.

Hell yeah it'll make a difference if you hook up a sub. I have a Klipsch SW10 II that has a 100 watt amp in it. You can hook it up to satts. mains or use the rca jacks to your preamp so as not to use any power from your amp. It is 10 years old and has about 1hr playing time on it. No BS I just don't need that much bass. I you want the sub I may sell it. Even though it makes a big difference, you notice it more when you switch it off after listening to it for a while IMHO.

Have you tried moving the speakers closer to the front wall?