Hi, I'm Mickie, I'm soon to be 49, have a lovely partner who has put up with me for the last 20 years, and I ride a 7.5FX (when my knee allows it) and a Trek 1500 WSD, I absolutely LOVE the 7.5 and wish I could ride it more.
Am I reading right that your knees bother you on the 7.5FX but not on the 1500?
If so, you probably just need some tweaks on the 7.5FX to fix that.