Never been to that specific one but old textile mills in general have a weird energy to them, I've noticed it in a few derelict industrial sites around South Wales as well. Something about the combination of machinery, repetitive labour, and probably some grim working conditions over the decades just seems to leave a mark on a place.
The basement thing doesn't surprise me at all. Low ceilings, no natural light, heavy stone or concrete absorbing god knows what over a century or more. If you're into spirit box sessions that kind of location can be genuinely productive, the acoustics alone make for interesting results even if you're a sceptic about the methodology.
What did you actually experience down there? And did you go alone or with a group? I'd love to hear more specifics before I say much more, because "something off" covers everything from a genuine encounter to dodgy wiring making the hairs on your neck stand up.