An old toilet cannot be retrofitted..It is true that you can slightly reduce the water consumption in an old toilet by placing a brick in the tank, but the savings is only about 1 qt of water and often it will cause the toilet to not flush properly... I am attaching two illustrations that I made. The first one will show the complete step by step process in how a toilet flushes... The second illustrates the technical differences between a 3.5gpf toilet and a 1.6gpf toilet. As you can see from the illustration, you can not convert a standard bowl to a water saver.

Are these new toilets that you had installed? Or were they there when you moved in? Do you know the make and model? I'm not a plumber, but a homeowner who installed three Kohler power-flush toilets many years ago. They were very loud. The Kohler ones we had were also quite unreliable and two of our three started having problems. Unfortunately, Kohler ran into difficulty with their parts supplier, so repair parts became either impossible or very difficult to get. So we ended up replacing all three. They were so loud that we found ourselves warning our guests before they used the one in the powder room. I think, in general, they are louder than most other types of toilets. Even if our old Kohlers didn't run into problems, we very well might have replaced them anyway because of the noise. Anyway, I don't know if they can be adjusted to make them quieter.

The Eljer was purchased for a number of reasons, cost (low), looks, and high seat, 17" off the floor. It has an elongated bowl. It is in the bathroom my wife uses, not me, I go in only to check it out every few weeks. She likes it, and she is demanding,so it must be good. I'd buy another, but Home Depot no longer stocks, but I do like their low cost replacement, which has good customer reviews. I forget the make, maybe Glacial Bay (if that's the name), a low cost toilet..and it too has a 17" high seat. The older I get the more I appreciate not having to stand up from a low seat. The subject application is for the "powder room" on the main floor. We have an American Standard in our master bedroom bath, my bathroom, and it works fine too, but is the usual low height. It is about 10 years old and the flapper started to leak so I replaced the flapper. The toilet operates differently now, uses almost all the water (more water) in the tank before the flapper closes. This gives both a cleaner and quieter flush...I like it. I plan to measure the tank to see how much water it is using. The arrangement uses a link chain between the release handle and the flapper. The chain has a float in the chain link and with the old flapper when the water got low enough for that float to let the flapper close, it snapped closed. The new flapper doesn't seem to be affected by the float. All this could be one of the problems with this "regular" flush design..but as I'm on a well, I'm not really concerned about a pint or two per flush one way or the other.

Homebound, The issue is the proximity to the living/dining rooms. I'd rather not hear the flushing noise when friends are gathering for a meal.