I use Tidy Cats Multiple Cats, but they also have a Small Spaces which is meant for places like ours (mine is ~700 sq ft and also just one giant room besides the bedroom). I've got two cats. I have found that it's not usually the litter per se that stinks up a house, but rather the places your cats urinate outside the box. My little fat orange girl kept peeing under my piano and I would infrequently detect the odor of cat pee - mostly when I'd been away for a few days. I didn't figure it out until I caught her doing it (never saw wet spots on the carpet, ever). I steam cleaned the carpet in my entire apartment and now it doesn't smell - I've even had third parties tell me it smells fine (also got my orange kitty to stop peeing under the piano).

You might sniff test your carpet or invest in a cheap blacklight to see if there are any hidden sources of urine odor.