I like the clear, lowest-chiff whistles too. I like the Tony Dixon aluminum high D, DX006. I was searching for clear, crisp rich tone in both octaves and after seeing it played in a few videos online, it jumped out in comparisons as having a particularly sharp crisp tone with no breathiness evident, the chiff level is extremely low. I haven't played every whistle made, and doing so isn't on my bucket list, but I've listened to at least 100 different whistle models online and owned about 12 different models, so I know well what chiff is. It may have it's place in some music applications, but generally I aim to have at least one whistle that's very low in chiff, so the DX006 suits the description.
Here's an example, among many online. Notice how it's razor sharp and the chiff level low. Any technical chiff seems to have been reduced and merged into a rich tone. It's the third of the four whistles played in this short demonstration by a talented player:
https://www.youtube.com/watch?v=aPVRbkMXJIcI thought the Killarney D, which you say you have, is already one of the lower chiff whistles. It's very efficient in air demand and known as good for session players needing long phrases and long hours of playing.
Looking forward to learning from this thread........