Sorry if I missed this earlier in this post; but, have you physically inspected the furnace and ductwork at its source? I never had a need to inspect mine; however, when I was adding rubber "feet" for my water pump, I did notice a seam or two had been loosely wrapped on my furnace. So, while I had the walls off in my basement (for the water pump mod) I re-taped my ducts. It greatly improved flow. I also removed my floor registers and pulled out large amounts of construction debris left behind from the factory. I used a shop vac to snake thru the ductwork for this.
2015 Seismic 4212 on order
2010.5 Dodge Ram 3500 MC CTD HD Dually (4.10 gears), Firestone Air-Bags, with a Superglide model 4400.
2012 Yamaha Zuma 125