Actually....... it's been proven many times that resin in your box WILL increase your spl. (most of the time)

The mics don't lie

 

Thanks Jordyo...I couldn't of said it better myself.

 

thats right meters dont lie : so here is my input to this topic .
I have tried fiberglass strands = increased in SPL
I did resin inside the box = Lost SPL
but it made a great seal inside so u know what happend i left it out side and it rained for a couple of days. So i remembered that i had the box outside and wanted to bring it in when i looked at it i was surprised i saw a pool of water the box acted like a bucket , the outside of my box had resin on the surface so it stayed dry also.
In the world of SPL its trial and error so many boxs i built loss with resin and few gained
Have fun and C ya

 

Thats good, so if your really into SPL, you could go the extra mile, and fiberglass your corners!