With 3 units I had very dark areas under 100 PAR and I ran 100% rb 100% b 90% w, now with 6 I got nice even coverage no dark spots, all depends on your taste. Even AI recommend more than 3 units for the 72x24x24...
Reefbreeders/Evergrow. Two 32" units. Under $1000 shipped.
I've had these for several months over my 125 gallon, and it grows and colors up SPS nicely. It actually bleached out a few LPS pieces of mine on the sand, as I was hitting ~380 PAR on the sandbed.