I tested no headrest this morning. I lowered the back of my recliner and sat up straight. No difference to me. But, it was a worthwhile test. My recliners (4 across) are thickly padded cloth. I suspect they simply absorb mids and highs. By the way, the top of my recliner is 40 inches tall (seated, my ears are about 38 inches). The midpoint of the AMT is 47 inches. I have felt no need to tilt the speakers down or raise my recliner up.
An interesting test. Thanks for letting me know about the issue.