Towels tend to get hard & rough over time. I've Net searched this some time ago and "cures" and numerous and varied. No idea if any work, some contradict each other. It was suggested that they can collect detergent over time and it is advisable to wash them in water alone once in a while, but I can't confirm that. One should never use fabric softener apparently, not that I do on any washing anyway. But if yours have gone rough then maybe it's time to replace ?
Believe it or not, leaving them on the line in the rain and letting them dry again has a softening effect. Rain water is softer than the water going into the washing machine and works a treat on towels.
I never use fabric conditioner on towels as it affects their capability to dry
mine are always tumbled dry as well. My mum liked to peg out towels on windy days as she was convinced it made them softer
I bought some new towels from Christy, they're lovely, no need to tumble dry them, I can line dry or of the weather's bad dry over a radiator, they're still soft and fluffy, I do use fabric conditioner.......
Debenhams used to have a big sign up in their towel department advising against using fabric softener. The reason for this is that the softener coats the fibres with a what's essentially a type of plastic, which makes them slippery. This leads to fibre loss in the towels in the washing machine and tumble dryer, and it also reduces initial absorbency.