I've always had cats and their poop can be very bad especially if they don't bury it. Not all litters are created equal. I ONLY buy Fresh Step or Scoop Away as those seem to be the best. I won't flush it since I found out that cat waste can was contributing to sea otter death so we use a small step can that has a closing lid. We put a small trash bag in it and it's ONLY for cat litter and it's near the litter box. I also use Arm and Hammer Litter Deodorizer.
The comment about food is right on. Cat food can make a big difference. Try switching to a good or different brand and add a little at a time to the original food so their tummies can get used to it before you switch completely.
I love kitties!