If the th600 isn’t bass heavy enough, but has too much highs you might be in trouble. Those were my kings for years. The th900 has a lot more bass, but even more highs. You may want to avoid those. My current love are the denon d7200, i feel these are the best compromise between both top end fostex models, with a bit of a twist.
Outside of modding/pad swapping the only option that may way work for you is the cascades. Also you say you are coming from iem’s, the bass is different for certain. It took me a while to get used to “less” bass of cans. I was an iem guy until a bit before the thx00 came out, so i know what you mean when you say the iem seems like it has kore bass. It’s the fact that its bass comes through a hermetic seal, directly into the ear canal.
I've just checked out cascades. These are the only basshead headphones for me. I've tried Sony Z7M2, Denon 7200, Audeze LCD2 closed back, Fostex TH-600/TH-900.
I'm going to sell my TH-600 as fast as I can.
Also, I liked CA Vega. They have amazing bass, but too much for my taste.
Did anyone compare Campfire Vega with IMR Opus Mia?