well first off a few questions so we can help more accuretly, so how old are they? have you started feeding with 1/4 strength nutes yet?
first off the larger 3 look like they could use a transplant, safe bet to say that if the plants leaves have extended to/over the edge of your cup then so have to roots, transplant time.
also the yellowing, depending on the age they could be crying out for food.
there could also be a pH problem, im not partial to pH problems as i grow organic and dont worry about it since the microbes do the adjusting for me.
humidity problem? i see none, the past month and a half since we started getting colder weather iv been down in the mid to low 20% RH and the plants dont seem to have even noticed, only down fall with low RH is when cloning youll need a dome of some sort to keep the RH in the 70%s, other then that they are absoutly fine.
as for the bulb, did you have the bulb IN the fixture simply turned so its off or did you compleatly take it OUT of the fixture? you may have knocked it taking it out/putting it in or if you left it in it could have gone bad from a partial connection for an extended period of time. and the 6500k is fine, thats the sweet spot for veg.
ooh side note, lower humidity could effect the wet/dry cycle of the plant, it could increase the speed at which the soil drys out so its not a big problem just stay ontop of things and keep an eye on them until the are in larger pots then it will be easier to keep them on a proper cycle, eariler in the year with around 50% RH i was watering about every 3 days, sometimes every 2 days during the really hot weeks, now with hovering in the 20%s RH im watering every 2 days. youll get used to the wet/dry cycle as your grow progresses. as ozzy said dont worry too much about low RH, its the HIGH RH that is the nasty nasty killer of crops, since at high RH both insects and molds/rots thrive you wanna keep it down.
