I have approx. a 4x4x8 with a 600. I too freaked out about low RH. Mine is running about 35 during the day...got as low as 25. I cut my exhaust with my light at night( I have a 6 in. inline fan shooting right to my attic through the ceiling for exhaust, and one passive intake at the bottom..no reason for a fan yet..my heat is hovering at 79 where the plants are) At night I have the inline fan cut off with the light, and have the humidifier on..so for 6 hours its probably at 60-80 humidity during veg. and drops to 65-70 temp.
My babies are growing great! Don't sweat it about low RH in veg, high RH in Flower makes them rot. From what peeps on here say.
My 4x4 is within a walk in closet, so I put my humidifier on the OUTSIDE passive intake of my tent..and RH is hitting 40% now. I don't know if this possible for you, but when I had the humidifier INSIDE the tent, my fan just sucked it right out.
I ordered a fan controller from Amazon so I can slow down my fan somewhat. Im happy with 45% no need for another humidifier to kick the power up. As of yet. It should be here soon in the mail, holla at me and ill let you know how it worked. I dont want to sacrifice high heat for high humidity. Keep them watered and humidity shouldn't be a problem..they pull moisture from the soil, not just their leaves!
http://www.amazon.com/gp/product/B003URBQ1M/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A19I28AKMOKQVB