Best in-ceiling speakers for audio

I think you are going to need to add a subwoofer for reasonable sound. I also think one of the biggest concerns will be getting reasonable sound pressure levels in such a large room with most in ceiling speakers. Theil makes a nice ceiling speaker with directionality, but it's rather small. I think for a room that size it might not be appropriate. Energy veritas I believe have some that are directional and probably a bit better in terms of power. B&W make some really nice in wall (could be used in ceiling though) in the CTX line that have the ability to point the mid tweeter to a pretty decent degree. These are quite expensive though.

I would also suggest that you investigate Mirage, who make omnipolar speakers. They make an in-ceiling version as well. It's called an "omnican". It goes in the ceiling cavity light a potlight, but there is a small half-globe which will protrude below ceiling level so that it can radiate sound in all directions. It looks a bit like the ceiling globes that hold security cameras in stores.

I'm running a pair of Triad 8" Omnirounds in my 10' by 18' by 8' high kitchen ceiling and they provide nice sound overall. Although they point "straight down", you can pivot the tweeter a few degrees to help with aiming the highs. Triad makes quality speakers throughout their line and are noted for their "architectural speakers" where placement or aesthetics is a challenge.

Their 8" size provides respectable bass, although you could add a sub like Rives suggests. I sometimes engage the "loudness" circuit on my receiver when playing them at modest volume and they sound suprisingly rich even without a subwoofer. One of the best features of the Omnirounds is they are designed with a closed back, which prevents sound bleed-through to the upstairs and provides a more predictable sound than open-back designs.

I power mine with a 100W Arcam 10P amp, but they are efficient enough that you could drive them with less, esp. if you have a powered subwoofer in the mix. Good luck with your system!

P.S. I used 14 guage 4 conductor speaker cable instead of 2-conductor pulled to each speaker to make sure they weren't starving since it was a fairly long run back to the amp., probably overkill but what the heck..