The only way you are going to get rid of the stinky poops is to change their diet, period. Get rid of the carbs.
Not all cats have a problem with carbs, but ones that do end up with anything from stinky poops to diarrhea and gas. A raw diet will almost 100% guarantee no smell, and small poops. A quality canned with no kibble is the next best thing. Cats are not meant to have carbs and their digestive system is not made to digest them.
Here are some good links:
A bible for all cat owners:
More technical data on cat nutrition:
Here are some great canned (all GRAIN FREE):
Wellness (does have some sweet potato which can cause some issues)
Nature's Variety, Instinct (no potato)
Organics by Nature
My occasional feedings:
Fromm's 4 Starr
My favourite Treat canned:
My favourite treat crunchie
My Jasper does not do well on carbs, he gets just like your kitties if he eats any amount of kibble or a bit of grains, especially corn then it is
. He eats half canned (the above ones mentioned), and half homemade raw and his smell is pretty much gone
. If somebody gives him more than a tiny bit of kibble, he gets very stinky