If a center pipe is casted straight into the base the CB will not function like its supposed to.
The hexagon combination of crystal pipes creates a vortex in the center of its field being pushed upwards.
If a 7th pipe is placed in its center it will block this vortex from spinning and the Cb wont produce positive qi like its supposed to.
A 7th pipe can be used but then it should be placed above the base itself, above the point where the vortex is formed.
The exact spot is hard to tell unless you are good at observing qi and it differs according to the size you build your CB.
A 7 pipe CB that has been tested quite thoroughly is The Torsion CB introduced by Laozu.
I have a good tutorial for it on my webpage that Rich and John did:
Here the 7th pipe is placed right above where the vortex is formed and it is channeled by the center pipe.
It works well without this pipe also.
I hope this answers your questions, I dont have a link from any discussion on the subject though perhaps someone else can provide that.