Sounds like you have a handle on the filtration & maintenance end. Most breeders, including myself, will stock beyond the 10 gallon per adult, 5 gallon per potential breeder guideline. With plenty of water changes, which you are doing, and overfiltration, which you have, you can double that stocking. Beyond that you have to have a backup plan, and be able to use it fast.
While Ken's fish is a good outfit, selling quality food and such, a diet of mainly flake food will cause some digestive problems. Frozen brine shrimp is an excellent food for adult angels, as the roughage keeps their digestive system moving. Angels are prone to intestinal protizoans, even more so with overstocking adults, as the close confines and pairing of adults can cause stress. Stress lowers any fish's immune response, the internal protizoans are considered a normal part of many cichlid's digestive flora.
What to do? Metronidazole sprinkled on some frozen brine shrimp will clear out the internal protizoans to a point that the fish's immune system will be able to deal with any remainders. Since it sounds as if they are having a bit of constipation issues, which may be increased by the killing of the internal protizoans I would add some epsom salt to the water. I would start with the epsom salt, one teaspoon per 10 gallons, on the first day. Bring it up to two teaspoons per 10 gallons the second day. On the third day, mix some metronidazole in with thawed brine shrimp. Since it sounds like you have quite a few angels, 100mg of metro per one tablespoon of frozen brine shrimp should be fed. I would feed this for several days in a row, along with having the epsom salt in the water. Feed once daily, and don't feed anything else.
Seachem sell metronidazole, nice little vial with a 100mg scoop included. Metro is something most all angel & discus breeders keep on hand, as this is a fairly regular issue with these fish. National Fish Pharmacy sells it in bulk;
http
/www.nationalfishpharm.com/products3.html
Once this clears up, feed less flake, and more frozen brine. Unless you are conditioning breeders or are working with a breeding pair fasting one day weekly helps to keep their digestive tract cleared out. Since they seem to be having a bit of a problem I would fast for two days in a row once weekly.