HLG-185H-C700 can output 286 volts. 10 Vero18s at .7 amp (asuming the Vero 18s are version 2) would want 287 volts when turned on... incredibly close, I suspect it would work no problem to run ten of them with it.
You could also run 6 each on HLG-120H-C700. 2 of those plus 4 A51 units would give you about 750 PPFD in a 4x4.
But since you have A51 Vero 29s already in the mix at nominal current, you should consider running the Vero 18s at nominal as well... that way you don't need to buy more A51 units. (Sorry EH
So, to get the light you want in that 4x4 for the least amount of money, here is what I would do. Two HLG-185H-C1050, each running 6 of the Vero 18s, along with the two Area 51 lamps, would give you a PPFD of over 850.
But passive is kinda iffy like I said at 30 watts, on the radial heat sinks I mentioned. Heatsinkusa has a couple nice profiles, maybe the 4.6 in a 36 inch length, two of those. On each run 6 Vero 18s with 2 fans, 12 volt driver for the fans, and HGL-185H-C1050. Would be close to $400 to get the rest of the parts you need.
And then you set it up like this:
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