My Behringer 'Power Pack 1' bass head & cab

HELP!

I have the Behringer Power Pack 1 which I purchased a few months back but havnt used that much (not had a chance to jam properly much). I seem to have a problem with the set up. When I play my bass through it, it easily distorts (sounds crap and the speakers kind of rattle). It does this when I play certain notes on my bass. I dont have the settings all turned up either for this to happen. If I use the 'Ultra bass' button (adds an octave lower to the sound)the cab can hardly handle it on a lower setting never mind turned up full!
Can anyone tell me whats happening here - is the cab faulty / broken? It makes it unpleasent to play through; as much as I love metal and agressive music I cant stand the set-up sounding like this - plus it is only on certain notes therefore just sounds stupid when jamming.
Please help, Im the next Jason Newstead.

daveyP