Thanks:). After posting I had a think about it and yes, if the spirit web is changed it makes sense for the Listeners to change. The spirit web is the person, filtered through the cognitive into the physical, so it makes sense that if the spirit is changed, this would cascade down into the physical, in a genetic way.
Crackpot theory here but... This makes me think. Could this mean that Listeners spirit web consist of a large amount of investure, and so are all Savants? Spiritual alterations can be passed down genetically so shouldn't savant-like traits also be genetic?
So, how I view the Unmade as symbiotic, they take something from a being while giving something back to said being. I believe Calderis theory, and how BAM works is to give access to the forms of power, but as a result takes/feeds off the spiritweb itself, so that when BAM was captured, it took the invensture with it. And if the Listeners are savant-like, could this be why they became SlaveForm, the investure taken was a large part of their souls. Dullform could have protected them because a spren is typically needed to "access" inventure. So if I add to calderis original theory, the purpose of a spren for Listeners is to access investure encoded their spirit web, and dullform is a result of a spren that cannot allow Listeners to do this properly. This could explain why dullform survived?
Is one of the purposes of the Everstorm a way to replace to replace this investure?
I have used a lot of conjecture here without evidence so I apolagise in advance!