Certain frequencies hurt

Hyperacusis. Possibly just coincidence with the cables.

https://www.leightons.co.uk/blog/hearing-care/sensitive-to-noise-misophonia-and-hyperacusis-explained

https://www.webmd.com/brain/sound-sensitivity-hyperacusis

Very interesting. I suffer from the exact same thing.

I find that I get pain in my left ear from my speakers as well, but very little if not at all from my headphones. It’s all new kit, Monitor Audio Gold 3005G, Michi X3, Series 2, Focal Clear MG. I know I have some hearing loss in that ear but the ENT guy has no answers for the actual pain.

What I also notice is when listening at very low volumes I find I need to tweak the balance to the left a bit to compensate but at decent volumes, let’s say around 80 decibels or so, the left seems to loud and painful and I can re center or even set the balance to the right a little bit. This will vary depending what i am listening to, be it jazz or rock.

But I find this effect to be almost non existent with headphones?

The ENT did mention hyperacusis during my initial office visit but after my MRI (unremarkable) he just dismissed everything telling me he can find nothing wrong..

Googling led me to the SCM muscle which apparently can cause all sorts of pain in strange places, including the ear. I had 6 sessions with a therapist and I found there was improvement but as my coverage for the therapy ran out I find the pain returning.

I just find it strange that it’s not as evident with headphones.

Would using a headphone amp with your headphones possibly help? Those Sennheisers sure don't need speaker amp type power.... 

+1 check ears first

Is the OP’s tweeters metallic? If so, soft dome might be better