Pics of what I have now. (The paper is sticking down because I was just about to light a fire when I remembered to take the pics.)
I do shut down for the summer, so that could have something to do with the U blocks crumbling. Maybe that is related to my issue with the refractory at the top crumbling too? If I can end this winter with enough wood left over, I may burn all summer for DHW. I can go almost a week without a fire with my large storage tank.
In the thread that leaddog linked, the U blocks are on the bottom, but I used fire bricks along the sides of them to channel all gasses to the front, then along the sides to the back. It made an improvement way back then when Zennon told me to try it. However, it made an even bigger improvement this year when I went to the new config.
At Barnartist's encouragement, I finally took off the front cover and adjusted my primaries this weekend. They were full open, now about 10mm. Secondary's were about 6 turns out, now about 2.5. Fan covers are open about 1/2 inch at the wides part of the pie. I basically kept turning down the secondaries till I got a nice gentle flame out the bottom. It still gets secondary burn quickly. It seems to burn much longer per load of wood, but heats the storage tank at almost the same rate per hour. I just restarted the boiler yesterday after being away for a few days, so I'll have a better feeling of how it is working as the week goes on.
One other thing worth noting - with my large storage my boiler sees almost the same conditions every day. Typically, the tank is between 165 and 170 when I restart the boiler, and the timer I use cuts it off when the tank is approaching 180. This is pretty much regardless of outside conditions. The benefit (I think) is that I don't see changes in how my boiler runs due to weather conditions. The only difference is that if it is warm enough I can go to one load a day, but the tank temps are still pretty much the same. So, now I'm curious - for others with large storage, do you have the same experience? Or am I just not noticing subtle differences?