17-19ft? And looks like no area rug on that (large) tile floor? Presumably, the side walls are pretty far away as well (for interfering sound reflections issue to be minimal wrt those walls)?
Makes me wonder if sound reflections along the floor could be problematic. And how high are the...
Besides what John just pointed out, what about speaker distance settings since you listed levels? IF you never checked/bothered w/ that, that could be (part of) the problem you're experiencing whenever something actually using/needing the full surround setup kicks in.
Beyond those, I'd...