with my 600w dim ballast from gavita i can go with 400w bulbs BUT my dim settings are 300w/400w/600w/660w. for using the 660w setting you are recommended to use a high quality HID bulb (they recommoned certain brands..dont remmeber who)
Anyways, i know i can use a 400w HID because the makers have it writen in the manual. so i advice you to contact the makers but unless they say it can be done dont try it! HID lights have about 5-10 atmopheres pressure in them... would not be a good day
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i recommend you getting a 600w buld and dim that insted.. when i think about it, why even bother with the 400w??
best regards