We don't wash diapers, but my husband's work-out clothes get pretty rank. We started adding Febreeze laundry additive to those washes, and that made a difference.
Unfortunately that is not an option with the diapers -- additives like Febreeze can really affect the absorbency. Plus I try to avoid things with scents, and it frankly makes me angry to spend this much money on a washer and then have to add things to mask smells that it is not removing.

I just made a habit to press the "Cold" button whenever I wash. I do that to save money on heating the water. Nothing has shrunk, though, even on warm. Maybe my water heater is not up enough (the former owner put a partition around it and I've never even seen the stupid thing).
Sounds like we have different versions of the machine -- if there were a "cold" button it would be easier, but it's that little button you have to press multiple times to get through the cycle of options. There is nothing wrong with it, it's just an annoying interface and I am irritated that "warm" is the default when most people wash on cold for the reason you point out, to save money. And our water heater is turned WAY down because we have a toddler who likes to adjust the water temperature in the bathtub.

I should stress that I am not comparing the Duet to a top loader. We have not had a top loader for about ten years. But it is nowhere near as good at cleaning clothes as our old Kenmore was. We really wish we had had that one repaired a fourth time rather than buying this machine. After two months, our towels (which looked like new in the Kenmore) look like old rags, and our diapers stink, and I hate this machine.

As soon as I can afford a new one I will probably put this one on Craigslist.

I hope yours works out, Irulan. I really wish we hadn't bought ours.