I just swapped out my Airports for Zyxel access points (within the last week, actually). I used the Airports for about 10 years, and my memory may be a bit sketchy.....

But if I do remember correctly:

The TSU9600's use WEP, not WPA (but I'm using v1 firmware on my TSU9600s, so that may have changed).

There was a version of Airport firmware that wouldn't play nice with the TSU9600s -- I actually tried it and it didn't work -- so I had to load an older version of the firmware. This issue may have been resolved in more recent Airport models and/or newer firmware versions, but I got it to a point of "it ain't broke, so no more updates".

Don't know if any of this will help you, but it may trigger some ideas.