They can be lower, but then you'll want to know how the speakers you plan on putting there function off axis vertically. It would kinda defeat the point if you put them 8 inches above your towers and they had poor vertical off axis response since you're trying to add the "tallness" to your soundstage by adding them. It's hard to see, but those are also angled steeply down at the main LP and I've got them crossed well above any bass frequencies so they never sound boomy being that close to the wall/ceiling (in addition to not wanting to tax the woofer). For me it made the most sense to put them as high as possible and also slightly out of position (heights are technically supposed to be directly above the L/R) because was having issues getting the soundstage tall and outside of the speaker placement, since those things are jammed in there. Right now it's even worse because they are set well back almost even with the entertainment stand to enable me to drop the screen lower.
If your front L/R throw a large soundstage the heights/wides are totally unnecessary, like when I was using the Phils. However, if a person's front L/R could use a little help for whatever reason, adding heights wides can be a cheaper alternative to upgrading.
To actually answer your question in a straight forward manner
p), yes they can definitely go lower and the ideal position for mine would technically be inside and even with/slightly lower than the THX/Panasonic signs. This puts them in line with the front L/R and keeps them away from the ceiling and any unwanted reflections. Speaking of, I also have the port plugs in, further increasing the bass roll off point and preventing any boominess or overdriving of the woofer.
