As mentioned, you can do it. But it will not be ideal, as the intention of adding height speakers is to create a more 3D soundstage. If all of your speakers are on the same vertical plane, this effect is virtually impossible to achieve. We have done plenty of systems with in-wall fronts (behind the screen), and in-ceiling surrounds AND height speakers. This is better, but still not ideal. You really want the LCR and side and rear surround speakers to be on the same vertical plane, and use in-ceiling or angled reflective speakers to acheive the height channels. The angled speakers work fine, but placing them correctly can be challenging, so I prefer in-ceiling.