I would not use a ball valve, just the height of the overflow, If you joined them together it may create a siphon in an area you dont want which may result in the overflow to stop. Or it may create more noise. in any case having them separate will allow you to individually adjust them to your needs. With the 90 My aquarium water level would rise an inch sometimes going completely submersing the intake. With the reducer, This allows more surface area for it to overflow at a faster pace It rises about 3/8 an inch.
75 and 36b reef tanks
180 new world cichlid tanks