I like the height of the speakers now, relatively speaking, as they seem to be roughly around ear level.
How much height do you have for a center speaker? (pushing the tv back, and having it on top of stand). If your Sammy is like my brother's, the IR sensor is smack dab in the middle, a stupid design choice if you ask me. I bought him the same remote system that I have, URC RF-20 + blaster, $80. I ran an emitter to the TV's IR, and you can't even see the wire, and he can finally have a centered, uh, center speaker!
JMO: The affordable (and often with compromise) center speakers are horiz MTM. Main viewing position could be happy viewers, but the side couch might get shafted with dialogue lobing. Ya know what's funny, people do run phantom, but they would recommend a center for the off-axis folks. IMO, however, I actually think phantom sounds better for those off-axis folks. The dialogue, and audio in general, is clearer due to lack of lobing. It might not be as centered, but I find that's usually not that big of a deal honestly speaking.
So, if you kept the speakers' height roughly at the same, and put the center speaker in front of tv, they will be actually very close to the same height. Some persons like to state a rule of thumb of within two feet, and yours would be even closer than that.
As for bass nulls, one thing that you can try, within aesthetic boundaries, is scooting up the couch a bit. The surround speakers then will be better placed, IMO (a bit more behind to straddle rear+sides), the TV will look bigger, and with a stroke of luck maybe the bass is better. You can even just try one-two feet; I don't think it will look too funny.
some thoughts.