I think it is important to determine exactly the type of "hole in the head" condition your fish have. If it is the type that manifests itself as tiny volcanoes on the head of the affected fish and you can see puss-like exudate, then it has very different implications than if you are seeing neuromast pitting - commonly referred to as hole in the head. The former is thought to be caused by a flagellated protozoan and the latter, is a physiological response to a stressor - usually something environmental. Given the conditions you describe, I'd suggest that your fish are being affected by neuromast pitting.
I'd further suggest that the reason for the pitting is that your 72 gallon tank is severely over-crowded. Given the physical space that such a tank provides, the tank could be considered fully stocked with just the three oscars. Also, feeding 3-4 times a day is excessive for adult cichlids. Finally, while it is good you have a regular maintenance routine, 40% water changes are not enough to prevent the build-up of waste products.
It is not surprising that you see pitting in the oscars and the chocolate cichlids, the two species are arguably the more sensitive in the collection you have. To arrest the condition, I would suggest you reduce the population in the 72 gallon down to three fish. Then, reduce the feeding to only 1-2 times a day and increase the water changes to 60-80% every time. Same treatment for the chocolates in the 55 - except reducing the number of fish - that may come a little later.
Also, while some swear by it, and some swear at it, I do not recommend that fish be fed beefheart. It contains fats that can not be utilised by fish. It therefore leads to fat storage issues, liver dysfunction and the excretion of wastes with very high nitorgen content - lots of food for the nitrifying bacteria to convert into nitrate. Other than that, the diet sounds great.
Lastly, healing the damage caused by pitting can sometimes be very difficult. However, with improved water quality and the addition of some appropriate live foods (such as small crickets) you might be able to heal the oscars and chocolates.
Hope this helps. Good luck!