What is the best ceiling mount for the 708i if I have a concrete ceiling? By "best" I mean the most safe/secure (i.e. no risk of the speaker falling down).
I would use an Omnimount : https://www.omnimount.com/products/speaker_mounts_and_stands/speaker_mounts/stainless_steel_series/25.0_c/
The critical part is making sure that the mount is attached to the ceiling in a safe and secure manner. I would make sure that your mounting bolts can handle at least five times the load you are suspending.