You can use a bio powder as long as your baby is ok with this, i use areial and its always been ok.
Your nappies sound like they need a strip wash. Sometimes hot water can 'cook' in smells. Try a cold rinse then wash on a hot washwith a small amount of bicarb. Then rise a few times on a cold rinse. Wash as normal with detergent and see if that helps.
Every time you wash your nappies try doing a cold rinse first so the hot doesnt cook in that wee smell. I strip wash my nappies about every 2-3 mths. If above doesnt help try wash at launderette so the big washers 'flood' the nappy with water. Good luck