Yea I've never used a dual spectrum bulb myself, I just mix a 600w HPS and a 400w MH in flower. How much do you pay for those dual spec bulbs? They look really sweet I'd run one myself if I only had the one setup I'm sure they perform a lot better in flower than just HPS
Halide grown Master Kush from Nirvana, about 30 days in, 12/12 from seed in my stealth-perpetual. Limited stretching due to the MH spectrum, (and 12/12 From Seed), despite being genetically programmed to do so from the Skunk#1 influence in this hybrid. 300w of MH Light in a small-space, I'm considering switching to the Hps just for the last week or 2 to induce that final Resin-Gush 
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I've found quite the opposite for increased potency. Hps seems to result in higher yield, but the intense violets and blues of the MH spectrum increase potency significantly in my experience. Personally I would trade yield to have slightly less, but a more potent product in the end. Also the stretch is minimized under halide lights and female ratios are higher due to the blue/violet spectrum. I started with regular seeds under my 12/12 from seed perpetual MH grow and about 75% ended up female. Just some perks when using the Metal Halide -
600w duel spec sunmaster bulbs r a reasonable....£20-£25......personally I feel they can compete with the better than average bulbs......but......if you move to the higher end bulbs that cost 3-4 Times as much obviously performance will increase......it's down to preference and space......
