How much space to leave for center channel

[Uniquely]

Sometime over the next few weeks..... I plan on modding my entertainment unit so that I can position my center channel under my screen instead of above it. To do this I will simply be moving the shelf that is above the tv down lower, and placing the TV directly upon that shelf (with some additional support of course) and then I will have the space below that shelf for my center. I can also build some supports to set on top of the side towers to raise the bridge that goes over the top of the unit..... to increase the total amount of usable height I have for the tv and the center. (see pics in gallery to get an idea of what I am talking about)


How much height would you guys recomend I leave below the tv for a future center channel upgrade? I don't want the tv to go any higher than necessary.... because I'll already be taking it a little higher than I prefer... but I don't want to severely limit my future center channel choices either.