I would imagine it starts with room size, but I'm sure there are other factors. I'm getting ready to upgrade speakers and design some new ones for my theater, but I'm unsure which method to go with. Waveguides and high sensitivity drivers will allow more SPL with less power, but they also change the directionality of the sound to make it more even. That could be a detriment in a smaller theater. So, what factors are key when deciding to go with waveguides vs. non-waveguides? Very few high-end residential speakers are waveguides, which makes me think that waveguides really only help on the super-large scale.
For reference, I'm talking about waveguide mids, not tweeters. I'll probably go with waveguide tweeters anyway.