It has been my experience changing cat food brands can indeed affect the cat's bm and associated odor.

What brand(s) do you use, dry or wet?

Take a look at the list of ingredients of your current catfood brand. Lots of brands use grain fillers and they frequently are listed in the list of ingredients -- in the first few.

I use a grain-free kibble food for my two devon rex's. Their coats have improved, less odor to the bm's and they run to the food bowls when it is time to eat.

I'm sure others will be by and offer their varied opinions as well.