Open it up, look at everything in the box, check the cross sections to see if there is silicone or something else airtightening it(might not be needed, but it could still be a possibility), then try to put silicone on the backpiece, so when you screw it back, it will be airsealed.
Check the speakerbaffle too, so that the speaker is tight enough/not too tight(This might be hard if youre new to it).
Also tighten the screws that hold the handles in place, they should be really tight(Do not try to tight them when it feels like the screw wont move without much force though.), and if there is something sealing the air from getting out through the handles, remove it, if it doesnt sound right, airtight it again with silicone, and if its not there when you look, tight it with silicone.
Also, if its on wheels, you wont get a really tight bass from the box.
Edit:
This video might give you some pointers: [ame]http://youtube.com/watch?v=Mze7-zhkx-4[/ame]