That's not good.
The volume of the speakers don't have to be set "visually" to the same level, even more because 4000's and 9's are not on the same league. I think that before buying anything, you should set up properly the speaker distance and levels.
1. Measure speaker distance and put correct distance from ear to speaker for each speaker.
2. Find a sound pressure tool. The cheapest is the free iPhone app "Ultimate Ears". As we are not searching for exact absolute levels but accurate relative levels of each speaker, it's really good.
3. Find a white noise source/go to the speaker setup to have white noise.
4. Turn up your TV main volume until you reach 75dB in Ultimate Ears at the listening position.
5. Go to the speaker setup and adjust individually all speaker levels to reach 75dB (front, rear, center).
6. You're done, now the speaker levels are correctly set up.
7. Try your new configuration before buying anything
[EDIT] Mistakes corrected