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?

1.  He didn't set it properly for no clipping.  That is physically impossible with music.

 

2.  Clipping can not damage a speaker unless the excess power created by clipping exceeds the thermal rating of the speaker.

 

3.  Tuning higher can give a "boost" in response making it a little louder but this can alter the sound making it undesirable.  There is more to designing a box than this.  You can also gain this boost by making the enclosure volume larger at the same frequency.  This is a more effective way to get boost from an enclosure.

 

4.  playing music below tuning the speaker will exponentially move easier below tuning.  As long as the subsonic filter is adjusted accordingly it will compensate.  

he adjusted it while playing a 40 hz tone but thanks for the help. would a 3 cu ft box for a single 12in fully loaded running on 1900rms make a difference?

Unless he used a analyzer with a switching filter he didn't. It's impossible to detect a undistorted or unclipped signal from a D class amp without one.

would making my box another .5 cu ft help with spl then? Im just not very satisfied as i thought this setup would be a lot louder

he adjusted it while playing a 40 hz tone but thanks for the help. would a 3 cu ft box for a single 12in fully loaded running on 1900rms make a difference?

Dude, with PROPER electrical your Sundown is doing 2k @ 14.4 just like my RF is doing 2k @ 14.4 .

Now I have same setup as you pretty much. I have a crap load of port area and am tuned at 31hz. With anything 27-30hz my 12 is crazy loud, and has fairly decent response up to maybe 45hz.

But hey that's what I wanted out of my design, "low end." YOU might not like your tuning.

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?

The above has already pointed things out that are needing to be addressed, but since you weren't impressed that also speaks that you have heard other 12's in setups that you were impressed with.  What were they?  If it were 4 and you are running one that isn't a relative answer...