Hi, for a while there I had that same problem and this was my experiance.
it wasn't the cage that smelled it was the room!!  i had a buera and they
could go under it and it turns out that they had been going to the bathroom
under there and i hadn't known it.  my ferret room is my bedroom so what i
did was took everything out, ripped up the carpet, and put down a linoleum
floor.  everything in the room was cleaned and i wash the floor once every
4 days.  plus, i change their litter box every other day, and wash ttheir
bedding (in my case 2 flannel sheets) every 3 days.  last, i use Marshall
biodor (so far i haven't noticed a change, but i've only been using it for
3 weeks) now they barely smell at all!!!
[Posted in FML issue 3446]