If the bottom leaves yellow first then it spreads upward, With droopy leaves and slow growth- it is a sign of soggy over-watered soil. Under-watered soil leaves droop but don't turn yellow. From what you say your plants are not over-watered. So i would suspect that it is nute burn. Personally i would transplant them into a soil with NO nutes in it and flush the soil. If you don't want to transplant yet, just flush the soil out really good letting water drain out the bottom. Wait a few days until the soil drys out then do it again.
Also the plants are short and bushy without much spacing in between internodes because you are using 6500k cfls (or 4000k but probably 6500k). Also because you have the light close which is preventing the plants from stretching (this is good). Those cfls are the right spectrum for the veg stage. If you plan on using cfl's for flower also, i would recommend switching to 2700k bulbs which have more of the red spectrum. (Red cfls cause plants to strectch compared to the blue you have now). Or you could just mix half and half which is the best option and will cost you less instead of replacing all the bulbs.
6500k aka cool white
4000k aka daylight (i believe) -stay away from these for growing!
2700k aka warm white ( has a orangeish-yellow glow to it)
Dense and bushy is good because the lower growth will get good lighting as well. I would even train the plants by bending the top over so the main stalk is horizontal, but wouldn't do this until after transplant. This way all the colas will grow upward equally and all will turn out the same size. Training this way is much better than pruning or topping because there is no recovery time for the plants.
Good luck.