For you guys using an MTM CC mounted below the TV, I've devised a little procedure that will help demonstrate the "seemingly insignificant details":
(It's real easy and quick, (it will take more time to read this post than it will to run the test), and it won't cost you a nickel.) Here it is:
* First, turn off any room correction you might have, but make sure your system is properly calibrated for level and distance. If you have subs, and you're using Bass Management, leave it on.
* Take a seat straight in front of the MTM CC. (If your theater doesn't have a seat there, temporarily place a kitchen chair there.) If you can, form an equilateral triangle with your seat and the L/R speakers, as that would be ideal.
* Now play some 2-channel, stereo content that has a solo vocalist recorded in dual-mono; Nora Jones, Come Away With Me
comes to mind, (if you're using the MC SACD, be sure to select the 2-channel stereo track, not the MC track.) Dual-mono means that the voice was recorded identically in the left and right channels. When this is reproduced by left and right speakers, and the listener is in the "sweet spot" the same signal will hit the ears from both speakers simultaneously. The brain will "hear" this as a phantom image, and Nora's voice will seem to hang in the space exactly between the two speakers. It will be at the same height as the rest of the soundstage and blend seamlessly. Close your eyes and the solidity of the phantom image increases even more.
* Next, move your chair 2 feet right or left. Now what happens to Nora's voice? The central phantom image collapses and her voice seems to come from the closer speaker. This is the "Haas Effect" or "precedence" effect. The earlier arriving and slightly louder sound from the closer speaker will cause the brain to "hear" the sound as originating from that closer speaker. This is why a phantom CC only works from the central listening position.
* Next, put your chair back in the middle. Start the track over, but put your system in PLIIx Movie, (or Cinema) Mode. This mode looks for identical content in the left and right channels, removes it from the left and right channels and it re-routes it to the CC. (Do NOT use PLIIx Music Mode for this. Music Mode creates a center channel signal from the identical content in the L/R channels, but it doesn't completely remove the content from the L/R channels, so it won't work for this procedure.) PLIIx Movie Mode will find the identical content of Nora's voice in the L/R channels and re-route it to the CC. Now what happens to Nora's voice? It changes pitch, timbre and tone because it's now being reproduced by a different speaker. The "image" also moves down and comes from a point below the rest of the sound stage. If the CC is near the floor, her voice will sound like she's sitting on the floor singing to you. Also listen to any instruments that were in the middle of the soundstage previously. They too will change timbre and image lower.
The exact same thing happens when you watch a movie with dialogue in the center channel. The dialogue will "image" from a point below the display, and it will have a different timbre than it would have had if it were reproduced by an identical speaker to the L/R's. In addition, anything that "pans" through the center channel will change pitch and direction, the same as the instruments around Nora Jones in her music. You guys didn't like my earlier example with the truck . Here's a better one: Put in Avatar and go to the night scene where the wolves are attacking. The wolves run through and across the front soundstage. In fact, they're all over in the "surround soundstage." You want their growls and barks and the sounds of their movement to be consistent for timbre and direction as they move around.
* If you want to take the above experiment even one step further, start Nora Jones over again, leave PLIIx Movie Mode on, and move your chair 5 feet to the left or right. What happens to Nora's voice now? It becomes hollow sounding, phasey and it's more difficult to get a good sense of direction from it. This is because you've moved into the off-axis "lobes" of the MTM speaker. The left and right mid-woofers are different distances from your ears. The sound from the further mid-woofer arrives a little later than the closer mid-woofer. This causes constructive and destructive interference which leads to comb filtering. ALL MTM's exhibit this issue, whether they are vertical or horizontal. They "work" better in a vertical arrangement because the lobes go over your head and to your feet. With a horizontal MTM, anyone sitting off-axis will hear this comb filtering.
The above procedure should help you understand the "seemingly insignificant details" of using an MTM CC mounted below the display. If you understand the compromises and you don't find them terribly problematic, then you're good to go. Please continue to enjoy your systems. If "pretty good" is good enough for you, that's fine. Or, if practical or financial issues don't allow you to use the optimal solution, that is also fine. Just realize the compromises you're making.
There are also some things you can do to reduce the compromises:
1. Mount the speaker as close to the bottom of the TV as possible, and as close to the same height as the L/R tweeters as possible. This will improve the directionality of the CC and the "lock" between audio and visual.
2. Use a horizontal CC design that minimizes lobing.
One such design uses a stacked TM in the middle of the woofers. This allows a lower crossover on the woofers and the lower crossover keeps them from reproducing in the range where the comb filtering is the worst.
Another design offsets the tweeter above the center of the cabinet and moves the two mid-woofers closer together. The closer together they are, the less comb filtering.
Another design uses a "tapered array" crossover where one of the mid-woofers is "tapered out" at the frequencies where comb filtering is the worst.
3. Use Room Correction, as most RC systems will make an attempt at timbre-matching the speakers.
Having said all this, I think still it's bad advice for a speaker designer or manufacturer to trivialize what is arguably the most important speaker in a Home Theater System. We get people coming on these fora all the time asking, "Why can't I hear the dialogue in my system". Or, "Why is the dialogue so low and then the surround effects blast me out of the room?" These are often people who have spent considerable $$$ on their speakers and that fancy TV stand/cabinet. Invariably, we find that these people have bought an MTM CC, mounted it in the cabinet down near the floor, they have hardwood or tile floors, and then they've placed a coffee table between themselves and the CC speaker. They've either received no advice, or very bad advice about HT setup.
A speaker designer, or anyone dispensing audio advice, should use, as their starting point, the ideal or optimal solution. Explain to the user why it is ideal or optimal. If the user then decides that the ideal solution won't work for financial or practical reasons, then explain the compromises involved in the less than ideal solutions. If the user ends up with something less than ideal, that's fine and it's on the user. At least they will have made their decision with all the best information. It seems Jim Salk does exactly that, and I respect him for it.