Hey labtech I got a question for ya. Given your experience what do you think the minimum height I would need to be able to do a three stack grow with a 600W HPS on the bottom and two more levels with 400W CMHs. Ideally it would all be one unit but I might do what you did and a 2 tier/stand alone unit. I figured with the two 400W CMHs I could use less height than you did for your 600 and also get plants closer to the bulb.
FYI, I tried the 400 CMH. It does produce more resinous buds, it does produce more leafy buds. I prefer the density and size of the buds I get with a 600 HPS. The 400 just can't compare. Just saying wattage equal, you might prefer the results of the 600's, and spend less money on equipment. If you already have the ballasts, then ignore me!