I wasn't trying to imply that there's something wrong with your hearing. π It was more to illustrate that our hearing range starts to deteriorate in our 20ies. It's quite normal for people in the age of 30+ to not be able to hear very high frequencies any more. In my case, there's not a lot happening any more the higher it goes above 16 kHz ... for some people I know, 12 kHz is the limit!
What I was trying to say is that if you're applying something rather subtle and only way on top, like 18 kHz upwards, the effect might not really strike you all that much. Try something in the range that's naturally the most important for us anyway - like distorting only 1 - 4 kHz. Are you still not noticing any effect? You should be able to notice a clear difference.
It would make sense to look into another set of monitors either way. The ones you are using probably do sound quite nice, but they're not really marketed as studio monitors for music production, but more towards a high quality music listening experience. I'm not familiar with that particular product, but it's quite possible that their frequency response isn't quite flat, but that they feature some kind of sculpted EQ curve to make the music sound more exciting. Which isn't really what you want when you're mixing music - you want to hear things as they "really" are to be able to make senseful mixing decisions.
It's still great to have them - even if you end up getting a second, more studio-oriented pair. It's a good idea to always listen to your mixes in different, more "consumer-oriented" environments, like multimedia speakers, car radio etc., to check if your mixes translate to the "normal" world.