I just saw and read threw this thread, and I am very surprised to hear about the problems with the 406 filters! I have 1-204 and 2-205, and they are awesome!!! Very quiet, strong flow, easy to clean, so no complaints here!
One thing I see different between the 204/205 and the 406 is the impeller cover! The cover on the ones I have are very hard clear acrylic, and they snap on both sides of the cover. On the first sponges (the tall ones in the plastic tray) the directions show to leave 1 side of the sponges sticking up above the tray. If I remember correctly it is the rear set. That is what keeps the water from bypassing them, because when I leave them sticking up like the directions say, when I go to clean the filter the next month the sponges are smashed down where they have been pressed down by the motor housing. I am not sure if the 406 is designed the same way, so take this with a grain of salt!
I am not trying to start anything here at all, but just trying to see if I can help! I really do hope Fluval isn't going cheap on us, since I have had my 204 for 7+ years now and truly love it! All I have ever had to do other than clean it was to replace the impeller last year, and I replace the tall sponges once a year too! That is it! I would 4 sure email or call Fluval and try to figure out a fix to your solution!!! Most manufactures I have dealt with are more than happy to get things worked out with their customers, because in this day and age a unhappy customer can spread the word to A LOT of people and quickly too!
So anyway BEST OF LUCK in getting this thing fixed!