By and large a sub is a good idea. Most 'full range' speakers are not quite full range. However, consider that many people run stand mounts happily. To my way of thinking either get a good sub or just run with what you have.
Two (or more) subs or just one? There is an overwhelming consensus that multiple subs are better than one. This obvious truth is unhelpful. The issue cannot be seen in a vacuum. First up, there may be space issues. Second two subs will cost more than one. I consider it better to get one better quality sub. On the other hand it may actually be easier to locate two subs than one. This is on the basis that you cannot get a perfect location for one sub. Now you really do need to get two subs.
Now, what brand of sub? I guess reputation and reviews and forum opinions are very helpful. Let me add a bit more: try to get a sub that will integrate with your system - ie integrate with your amp and your speakers. Here are my tips:
First, if possible get a sub that takes high-level input. That is to say it takes the signal from the power amp rather than the pre amp. (Not a big deal if you have an integrated). The reason is that your sub and your speakers will share the exact same signal. This is better because the signal from the pre-amp is a little different than the signal from the power amp. This is strikingly obvious if you had a solid state pre and a tube power amp.
Second, try to match your sub and the power amp. If your power amp runs SS with mosfets, then a sub with mosfets would be ideal
Third, try to match your sub with your speakers. So: paper cone - paper cone; carbon fiber cone - carbon fiber cone (what I have). Ported design - ported design; sealed cabinet - sealed cabinet.
Best of luck
