To much light, to little plant!! and your spending too much money....
600W HPS=140 lumens
1000W HPS=120 lumens
and your adding heat which will give you the tall, stringy, airy buds.
instead of "substituting blue spectrum light" with your plants (which is why im thinking your wanting a 1000W Metal Halide IN ADDITION to the HPS, why not just substitute your 1000w HPS with some UVB (suntanning lights), they will give your plants the right blue spectrum from the MH during the flowering period that will make your plants yield higher and have a higher potency.
But the stretching of your plants is because of the strength of the lumens is too low so your plants are stretching to get more lumens(this is why 600W HPS is the best bang for your buck) yet with that stretching, your plants cellulose and potassium levels are being sucked up by the stalks of your plants for further height instead of bushy out and producing more buds.
after you fix your substitution of lights whether it be UVB or with 600W HPS (UVB is cheaper and better)
Start a constant occurance of topping your buds....trim the tops so that the cellulose and potassium pushes outward towards bushing your plants then you should be good.
keep me updated and send me more pics and questions and ill do my best with all my references available.