If you're serious about movies, do look into a matching center channel amp/speaker. For movies the center channel speaker the most important speaker in your system. It contains the lion's share of sound and nearly all the dialogue. Some will say that two speakers (particularly if they image well) can substitute for having a center channel, but it just ain't the same (even if that center channel speaker is fairly lame). And matching the tweeters (called timbre matching) is critical to getting a good blend with the left and right front channel speakers (far less critical for any rear channels to timbre match as they primarily only pick up ambience/special effects).
Against common thinking, I'd recommend against the typical MTM horizontal arrangement for the center channel. Two competing mid/woofers smear (identical signals that don't arrive at the same time will be out of phase with each other) when listening at non-equal distances. For vertical MTM arrangements (like your Song Towers) this means listening at mid-height between the drivers (tilting the speaker can adjust for listener height). But horizontal MTM means listening only directly in front of the speaker. Put together vertical and horizontal MTM would result in only a very small sweet spot (like literally being in the crosshairs of rifle scope). Since home theater is often enjoyed as part of a group, this becomes more important. I've tried a small MTM speaker on the center channel versus a slightly larger 2-way (turned on it's side to fit) and the 2-way smoked the MTM (both from the same manufacturer).
So my recommendation to you Frank is to save yourself some money and space by having Salk build you a single 2-way SongSurround.
(mresseguie, you should have installed in-wall speakers for those rear channels and painted the grills to blend with the wall)