What do you mean what do you need to know?
There are different motors - further down the alphabet means newer, g series is the newest. Most people agree that there are pros and cons of each, but most people like the f and g motors fine.
9900 series is no more, there is the 99z series and 9500 series. Any 9900 series subs that are available are older, and are simply a 9500 series sub with a composite cone (IIRC).
Other than that, there isn't much to know. Look for a 9515f or 9515g and you'll be fine.