The "over the internet diagnostic" is asking alot and subject to many variables, I know. But many of you have years of experience and a good understanding of the science at hand, which makes your tactical guesses better than what I am paying for, I hate to say.
I went back to the install and studied it. The only other option where there is room for these needed take-offs, is to come directly off (downward)of the 8x20 trunk/plenum below the downfeed AH. Currently, the trunk design is such that the AH base plenum downfeeds directly into the 8x20 trunk. The trunk goes "right" to supply the downstairs where the "sleigh" transition would be located (if used). The direct opposite (left side of the AH base/plenum), has the 8x20 trunk immediatly transitioning into a short side 90 and going up through the sub-floor alongside the AH to feed the second story. Alongside the AH the 8x20 transitions into a 12" round to continue to the second story attic for 6 cieling supply registers.
There is 11 inches of clearance below the 8x20 trunk under the AH. Is it concievable to use this room under the 8x20 for an extended plenum to be used for take-offs?
There is a plumbing drain pipe located in this area currently, but I can have the plumbers re-route this.
If so, what would be the correct way to integrate this extended plenum?
I hope my description is making sense!