Unfortunately, the products you've been using will not eliminate pet odors......they just mask them.

You need to use an enzyme product made for cleaning pet urine/feces. I've used the one below and it works. Most pet stores sell these enzymes and they're also sold online. Look for anything recommended by dog/cat kennels!
In addition to using the enzyme I'd recommend you seal the concrete after it's thoroughly dry. That should take care of the odor in the concrete. If the cat has sprayed the walls and the odor has seeped into the wood then that odor will be a little more difficult to get rid of. The OdorMute listed below can be used on wood also so I'd probably spray all the walls and the floor during the cleaning.

I'd strip the basement bare since it's possible the wood and fabric on any of the remaining furniture has absorbed the odor.

While showing the house you should move the cats and their litter boxes to a spare bedroom or the garage and let the realtor know that no one is allowed in those rooms during showings because of pets. If you have a serious buyer, they will return to see the room they couldn't see on the first visit.

When showing a home, it's important to remove the pet and as much of its stuff as possible. It doesn't matter that you keep the litter boxes clean if the cat has decided to do their business right before a buyer arrives to view the home. Plus, people who don't own pets often are more sensitive to odors that pet owners are accustomed to.

Here is a link that might be useful: enzyme cleaner

I have two suggestions, both I've tried for various situations and both work wonders, from a locked up vacation home that had a mildew smell after too long to bowl-you-over perfume smell saturating the drawers of an antique bedroom set. It will take a week or so to kick in, but then you should notice it. Doesn't mask odors, but rather absorbs them.

Citrus Magic or my preferred Natural Magic gel tubs. Then there is Gonzo volcanic odor absorber, they have one for basements. You can reactivate the Gonzo bag every 6 mos by setting it in hot sun for a day. I'd suggest you do that first thing when you purchase it, just for extra measure. You should be able to find all these products at a hardware store (Home Depot carries them). And I would use both Gonzo and Nat Magic tubs at the same time to really get to work on the odor. Agree with annz on cleaning with an enzyme solution as well.

Have you tried running a dehumidifier down in the basement as well to dry out the damp air? Drying the air will help with some odor as well, especially after you've scrubbed so much, there will be dampness in the concrete, etc.

Nature's Miracle (PetsMart) worked to get stains and smell out of our carpet. You might also consider cleaning the concrete floor with TSP, but follow the directions precisely in a closed area like a basement.

Nobody suggested putting plastic sheeting down under the litter box to catch future accidents.

Lots of good cleaning ideas. The dehumidifier is a good idea. I have a truck that smelled like dog in wet weather for a couple years. It may be bacteria making the stink, and they go dormant when it's too dry.

I was thinking about an ozone generator, maybe you can rent one and put it down there for a couple days.

Regular Nature's miracle won't work for cat stink. Try:
Pet-tastic, Simple Solution, or Nature's Miracle Cat Formula.

My local Pet Supplies Plus supposedly has a product specifically for removing smell from concrete - but I don't know what it is or how it works.

I'll second the idea of a dehumidifier - it's amazing how much stink comes out when the weather is muggy.

You can also try a new litter 'Cat Attract' - it has something in it that makes cats want to use the litter box.

I used Kilz (it's like thick paint) in my last house on the concrete floor before we laid new carpet down. Worked like a charm. Ventilate well.