Additional speakers are not what is required. Because you are going to insert the port bungs into the speakers, you are going to struggle to get a frequency response below 50 to 55hz. A sealed subwoofer will fill in the sound nicely and it will help make it sound great. For television, I find a subwoofer is more important than for music.

I would look at an NHT B-10d for $599 or a Sunfire HRS-8 for $850. I have two Sunfire subs paired with bookshelf speakers in two different systems and am extremely please with the results.