I wouldnt buy gravel for there if you can help it, just any old stones from the garden will do for drainage as a layer over the soil, before laying the turf down again. Could be the synthetic fibres are soaking in the smells, give the biological cleaner a chance, I think that should help (you may want to do as above with the cleaner but allow to cool and then soak the turf in it if you have an area or large container? then allow to dry before replacing?

Edit - if you need to buy gravel, dont ask for the cheapest gravel, just ask for a small amount of type 1

Also, softener may just cover smells for a bit? Biological laundry powder or detergent will break 'dirt' down better I think.

Also, maybe putting a bit of sand in might help?

Also, once you have got rid of the initial smell - at my last house I had a chipped area that the dog used and once a week I would chuck diluted Zoflora down at night once he was in and never had a problem with smell there.