Here are some things i'd try to make sure I did:
- Keep the drivers vertical
- If at all possible, have the tweeter at ear level
- Keep the individual drivers as close together as possible
- Instead of lining the walls, you can also use something like Acousta-Stuf
- The covering for the speaker itself should be as sonically transparent as possible.
- Flush mount the drivers to the front baffle
- Make sure that the drivers are flush with the front of the credenza -- don't have them inside at all
- Depending on the size of the speakers cabinet, you may want to brace the inside.
- If your speaker has a BSC built into it, you may want to remove it -- it could mess up the rest of the crossover performance, but it might not.
- Make sure the speakers are equidistant from the tv
- Spread the speakers as far apart as possible