Fan placement for optimal air circulation

[markem]

Depends on where your air gaps are going to be.

For large cabinets like yours, I would think that vertically positioned but located at about half-way up the back corners would be good. This will both draw up air and also blow out air, which should give increased air movement. I've also seen opposite corners where one blew up and one blew down, but I think that this causes air gaps in the center.