We don't own a dryer and after two and a half years the only thing I've found that really makes line dried towels not so crunchy is Downy liquid fabric softener (April Fresh specifically).
I know, I know! Not at all natural. And yes, they are still crazy absorbent. I don't know if the fabric softener + towels = non-absorbent is an urban legend or maybe it's liquid softener + driers that's a bad combo, but anyhoo it works.
I wish the "natural" softeners worked as well, but IME they just don't. I've tried Ecover's liquid softener and it was pretty terrible. I'm currently trying Vaska's version and it's definitely better than Ecover but not as good as Downy. Also Vaska has a funky consistency to it and I think it would gunk up my machine if I used it too much (Downy is not at all gummy and has never gunked up my machine). I'd like to try 7th Generation softener. Has anyone here tried it and like it?
Also new towels stay nice and soft for a good while with or without Downy, but definitely better with it, it even helps old crispy crunchy towels
Something about that darn animal tallow that natural softeners just can't replicate.