currently running a sundown saz1500 powering a fully loaded 12in fi bl in a 2.5 cu ft box tuned to 34 hz. 

I was recently at a local audio shop and had the main guy set my gains properly so i did no have any clipping. I was not very impressed with the outcome. As i come to find i had been running with clipping since day one and my system is now quieter. He told me to tune my box to 40-42hz and it would be louder. My question is how much louder? and will i still be able to play most rap music? What would be the lowest tone i could safely play?

i would not tune upwards of 40. i did with mine and hated the outcome. had no lows. try tuning even lower. if you insist on a higher tuning, go with higher 30's first before going to 40's.

i have the d2 version and the fs is 39 so does the fs of the sub have anything to do with what i should tune my box too?