I'm thinking since you've been fighting thsi problem for this long, you've done your research and tested all your levels. I had a similar problem with the water always going cloudy. I accidentally fixed it and it hasn't been a problem since. I was trying to treat a phosphate problem (testing off the charts) so I took phosguard, stuffed it in portion of nylon (1 pair makes many sacs) and secured it over the outlet spray bar for my Eheim 2213 (60G) canister filter. By the next day the water was crystal clear. I looked at the nylon that was over the outlet bar and saw the inside was filled with tons of tiny, tiny crap that had been bypassing my filter. I then took an algae magnet and scrubbed furiously to clean crap off the glass. Again, the nylon in a short time caught all the floating crap. I only had the nylon on for a few days before I packed the PhosGuard into my canister, but I'm definately trying it again if it clouds up again.
If you one of your filters has something like an outlet bar (rather than a HOB), perhaps try securing a nylon over it and see what it catches overnight. On the bright side, if it does nothing, at least there were no chemicals added...
Colin