Your dehumidifier rating at 50 pints/24hrs should be well suited for the size of basement you have; above and beyond actual size is the efficiency of it, but that's a moot point. Some experts say dehumidifiers are most useful down to an indoor temperature of 65į F; temperatures below that cause some units to ice up, in addition to simply working less efficiently and wasting money. But all things being equal, your unit is doing what it should. How much water do you remove physically every day? a full container, or just a cupful or so?
You also seem to be doing as much as you can about the flooding in installing French drain and the sump pumps; that's excellent. The french drain system and the sump pump which are connected to it will take care of water underneath your house. If the sump pump pits regularly fill with water and are then pumped into the French drain system, that is what is happening and your basement will become drier with time. Of the 5 different methods by which moisture can enter your basement, you have taken care of 2. Next in line is that which is coming in via the walls.
Please note that we are not trying to turn your basement into the Sahara desert - just trying to keep the moisture level in check. Ideally, and most practically, the humidity level in the basement will be around 40% on average year-round. Less during the winter (say 35%) and more during the summer (say 55%). A cheap hardware store humidity guage is most useful here. But these levels will be slightly higher (by 10%) than the rest of your house, where there is hot air pumped around by your heating system - but less moisture coming in. So one way to dissipate that moisture is to have the fan on your furnace if you have one on all the time. Better yet are fans down there not necessarily from the outdoors in, just blowing the air around.
To bring in air from the outdoors, you'd have to turn on the fans when the RH outdoors is less than that indoors, so you'd have to either follow the weather forcast constantly, or get a guage for the outdoors, and the turn the fans so that the air is either expelled or brought in, depending. That's a lot of playing for most people.
The fact is that at 60% RH, moisture will promote mould growth if the air is still. Keep that same air moving and the chances go down. Lower to the RH to 45% is a good step to making sure it never happens...
OK; so now you're like millions of homes: manageable RH in the basement. Now you can set your sights on the walls where probably half the moisture is coming in. An easy DIY thing is to insulate the walls with the 2-inch pink T&G polystyrene boards (2'x8') glued together and glued to the walls, once the wall are made flat. Tape the joints with that red tape and bring the boards down to about an inch from the floor. Insulate the cavities between the floor joists with fibreglass insulation and your temperature will go up down there by about 5-10 degrees F. Then perhaps do the floors...by heating up that space, the RH will go down a bit and away from the danger zone, making the measures you have already taken more effective.
Ultimately, you may not need to run the dehumidifers as much; that saves you money in the long run, probably enough to pay for the foam boards you've just installed...