My suggestion is that you change out the cage every 2nd or 3rd day (4 days is probably too long to go) and add vinegar to the rinse every month or so. The laundry detergent does sometimes make a difference. You might try Tide. If it's expensive for you, just use it for the pig stuff. Another thing that may help is to go to a 2X4 cage. A 2X3 is the minimum requirement for two females, but I think a 2X4 should be the minimum. The larger the cage, the cleaner it will be.
If you are expecting babies however, you may want to use shavings during the time the birth may happen as it's easier to spot clean the cage after the birth. You can always switch back to fleece afterwards (don't forget to baby proof the grids beforehand too).
For my two pigs I have built a cage made of coroplast and fleece; on the coroplast I have put a large towel, a layer of newspapers and just in contact with the fleece I put a lot of those advertisement flysheet you can find at supermarkets. In the morning there are a lot of droppings and wet zones, but no smell. Anyway, I hang out the fleece on the sun; I sometimes change the flysheet or I turn it over and I let the pigs on the floor until the fleece is dry (or I put a second fleece on the cage). When the cage is ready again with new flysheets and fry fleece there is no smell at all. I wash the fleece maybe once a week together with other laundry, with normal soap and no softener. I think there are fleece fabrics which are not good for our use. I bought a large piece of good fabric and I cut it in two pieces. Maybe the smell depends on the food. I don't feed my pigs with pellets, only hay, grass and vegs (and two drops of vit C).
1- buy a piece of fleece fabric (here in Italy the best ones cost not more than 6€ a meter) and under it put newspapers.
2- if it does not work, buy an urine stick and check what is happening because smelling urine can mean an UTI or some unhealthy condition (a wrong PH) which can lead to the infection or something else (also in children and adults).
If everything okay, throw pellets away. (The two pigs belonging to this lady are 8 and 9 years old! they never ate a gram of industrial food).
It is not normal to have such a smell at home...
Lynx is right about the hay. Soiled hay is very smelly. If you are pulling that out when you clean up the poops, that might help too.
Another thing you might try if you haven't already is to make smaller fleece pads to put in the places where they sleep and spend the most time. I made them out of a uhaul pad sandwiched between two layers of fleece. These I change daily or more to help cut down on the smell.