Hirschman KR Indigo Trumpet Plunger

The internet has some very positive reviews of the KR Indigo plunger. However, when it arrived I soon became disappointed with the result. I tried it with my Thein Berlin, a professional cornet. I mostly use mutes by Jo-Ral (copper bubble and straight mute), and Humes & Berg (cup and solotone), which I like a lot.
This mute however, not so much. It does not fulfill its promise of not changing the pitch when you play in closed position. Notes change at least half a tone in pitch. The lowest notes a full tone. You really need to maintain a large gap between the bell and the plunger to prevent this from occurring. But then, if I maintain that gap, it does not sound closed and I am attacked by my own sound. This does not happen if I use other plunger mutes. My plunger from the hardware store does a better job.
Maybe this has to do with the interaction of the plunger and the rather small bell of my cornet, which is just 12,2 cm in diameter. The plunger did not seem too big, though.
I also did not really like the way this plunger feels. It is too wobbly to my taste.
Kudos to Thomann for the service, though.