Heat is a limiting factor. High pressure sodium are the biguest culprits. The temp at your canopy should be between 73-78°. Also, have you noticed less frost on the tops, under hps? The heat causes the plant to use it's resins as a defense against the heat. Hence the smell you probably have, even with a filter.
Try backing the light off some. I have a 1,000 watt 36" away from the canopy, and have better bud now, than when I ran it at 24". The lighting industry won't tell you, but the footprint of big lights isn't just what they say. Question it.
When you reach temps in the 80's, you risk hurting your plants. Transpiration is necessary for growth, and this stops in excess heat. Do you vent your scrubbed, hot air?
Also monitor temp under the canopy, make sure air circulates down there.