Please, help me to pick the right center and surround speaker for my home theater

I have my current set up:
- Integra rdc 7
- B & K reference 200.5 
- Martin Logan Ethos ( L+ R seapker)
- I used to have Martin Logan center cinema i ( but sound not right) sold
- Definitive mythos gem for surround ( just want to upgrade new on, please recommend)
Please, help me to pick the center and surround speaker. Thanks a lot...
Generally speaking, it is important to keep all speakers in the same family. When you hear a soundtrack of 5.1 or 7.1 or so forth the sound effects can move from speaker to speaker. Think of a missle sound going from the front speakers to the back ones. The tonality and characteristics of sound should stay the same. That is why home theater systems speakers systems are sold as a package often, but not always. Most have a center channel and low frequency speaker (driver) subwoofer to enhance this theater experience and the speaker cabinets have the same kind of drivers.
You know my secret source for center channels is Parts Connexion.
They always have closeouts on good centers.
For 600 bucks you really can't go wrong with an Aerial center speaker.  For surrounds, well, not sure how much you're gonna notice an upgrade there.  Maybe some Elacs?
Thanks for all the respond.
-  I will buy the martin logan motion 35xt for surround,
-  I try the martin logan center speaker esl c but the sound still not loud and clear. I still need to try another center.