Center speaker? Why would anyone use a center speaker?
Kidding. My center speaker can't be moved unless I mount it on the wall above the TV. I don't want to do that. I thought about sticking a couple of rubber door stops under it to angle it up, but I haven't found the motivation yet. I think it sounds great where it is. My ears are only about 9° above it when I'm sitting, which doesn't seem to be enough to affect the sound noticeably. If you're curious,
this is what my center speaker looks like without the grille.
My sub is behind the seats facing a corner. Behind the seats is the only place there's room for it, and facing the corner is what sounds the best. I've also got Aura Bass Shaker Pros mounted under my seats, which are great fun for movies.
Only real complaint about my sub is that its auto on / off isn't as sensitive as I'd like.
When my receiver is at doctor's office volume level, my sub doesn't always turn on. I've thought about replacing the plate amp with one that has a defeatable auto standby, but it's always been too low priority for me to address. My hackish workaround is to turn the sub's gain down below 9-o'clock and turn the sub channel on the receiver up to +6dB.