Thanks, Martin. I had a hunch that we were being hampered by vertical depth as our smaller mashes run without issue.
However, the permeability of our large mashes decreases over time for some reason. Our water/grain is 1.25 for all of our mashes. Next weekend we re brewing a doppelbock using a 41# mash bill (not including rice hulls). Through our research it's recommended that a doppelbock mash water/grain should be around 1.4, so we've made that change to our calculations.
For us it's a tug-of-war between sacrificing mash efficiency to make bigger beers or retiring our mash tun and investing in one that will allow us to brew bigger beers with a shallower mash depth. We're hoping that the integration of rice hulls into the deeper mash will allow us to improve the mash permeability.