As with many things in the audio world the answer is a resounding, maybe. It all depends on a few things:
- What type of music are you listening too?
- Is this a home theater setup?
- Is this background music or are you sitting and listening?
- How much do you like heavy bass in your listening?
- If this is a home theater setup, yes, get a subwoofers
- If you're only using the speakers for background music, you likely don't need the sub
- Depending on the type of music you listen too it might not require deep bass to be satisfying
- And of course if you really like lots of bass, you already know the answer
- Do you have room to put another box on the floor? (only you know this answer)
Proper installation is the key here however. Also consider closed back speakers for more accurate sound.
You can in most cases add the subwoofer later depending on the location of your amp. In either case if you purchase high quality speakers, you're sure to get years of enjoyment out of them.