I've read someone online using these words and I use them to describe my experience, but that's probably something totally made up lol

Passive Soundstage is the feeling of the sound being wide because of the open back nature of the headphones, simply because of the fact that your ears aren't occluded and the drivers are further from your ear canals. For example, iems and closed back headphones lack this feeling, which I describe with these words.

Active Soundstage is what people usually refer to as "regular" Soundstage, and is the feeling of the spatiality of the sound despite the actual distance from the driver, ear occlusion and so on. Imo it depends on both tuning and technicalities like detail retrieval, layering and "imaging", and from a tuning perspective it depends on how the headphone/iems follows your particular hrtf.

My experience with the ksc75 is that yes, they feel open compared to an iem or closed back in the same price range, but though being open the sound feels quite "in your head", and there isn't much separation from instruments being in the front, in the back, closer or further away. Imo that has to do with the headphone struggling with layering, in a way that if two different instruments are playing at the same time, especially when there's a difference in volume, a part of the detail of the sounds is lost and therefore the illusion of space is compromised.

Instead, with the venture electronics monk plus, to make an example, the layering is much better and that in my experience results in a much more accurate Soundstage and feeling of space, even though the specific location of the sound (imaging) is not always clear, so that results in sounds in the "background" as sounding further away, while still detailed, and sound in the "foreground" not being affected by the other sounds around them, resulting in a clearer and easier to understand depiction of the music.

Another example of "passive" vs "active" Soundstage is an iem like the Moondrop Aria. Being an iem, your ear canal is completely occluded, and therefore they lack that "open" feeling of open back headphones or flat earbuds, though they make up for that with technicalities and tuning, that are pretty good for the price. That makes the sound still feel "wide", especially stuff sonically designed to sound that way, like ambient music, or well mastered orchestral music. The spacial image is more defined than the ksc75 Imo, though being closed they feel wide in a "different way", therefore why I started using this comparison between these different feelings still using the word "Soundstage"

Sorry for the wall of text, also this is just my personal way of describing my experience, and it must not in anyway be taken as something universal or that I expect anyone to agree with. There probably are people with better knowledge and better terminology who can describe what I'm experiencing better, this was just my personal take on the subject "Soundstage" lol

Always a silver lining. Love my DT-990s so much.