Stiff towels usually mean too much detergent, not too little softener. Use about half the amount on the box and add an extra rinse.
Wash towels at 60°C, never fill the drum more than three quarters, and shake each towel out before it goes in the dryer.
Fabric softener coats the fibres and makes towels less absorbent over time, so we leave it out unless a customer asks for it.