I need more hot water

Actually this is more of a question as to why my gas fired furnace with domestic hot water core seems to run out or at least to warm (instead of hot) before the furnace kicks on and brings it back to where it should be. Once the burner is running the water is heating up after 1 minute (if the faucet is off) or 2-3 minutes (if you leave the faucet on and continue showering in the lukewarm water) . The shower heads are the 2 GPM rating and I am told the DHW coil should feed roughly 6GPM fine. I have changed the water temperature "mixer" valve and that seemed to help some but it still runs too cool before the burner kicks on.
A while back the thermostat control box and temperature sensor was changed. That control box has 3 dials marked Hi, Lo, and Diff. The Hi and Lo dials are set to the burner off and on temperatures (obviously). The reason I mention it is one is set to 190 and the other is set to 170 and my core temperature guage seems to read these values when the burner cuts out or turns on. So they seem properly calibrated I guess.
The Dif dial is at the zero mark. With an on and off already setable I can't believe this is some sort of calibration like one would have on a scale. No real need since if you need the core to be at a given temperature you can use the guage and increase or decrease the Hi/Lo setting to get the desired temperature. In other words if you need 190 and are only getting 185 turn the Hi up a smidge to whatever it needs to be until the GUAGE says the core is at 190. The numbers n the Hi/Lo are really only for reference and the guage is the actual temp to go by. At least that is my thinking.
So is the "diff" some other on/off control and does it affect my hot water output? Rather than experiment I figured I'd ask here and suffer the slings and arrows from the "hire a pro" crowd but the last 3 guys doing service or yearly inspections have been asked and never really answered the question. Two suggested a third zone to heat a 30 gallon tank and the other suggested an instant on wall hung hot water heater. Yes both will solve the problem at quite some expense. And that is why I want a simple answer first to the question and then I can start saving my pennies if need be.

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