Distortion and crackle sound with laney bass amp

[adam7432]

HI,got this laney rbw200 bass amp from someone.The amp had this distortion and crackle sound when play.The note could still be heard but die off quickly!
Need someone to direct me the problem.Is it the power supply, power stage or the preamp.
Could any kind soul here help me to solve the problem.Below is the schematic for the amp.


http://www.tremolo.pl/Firmowe/LANEY/RBW200.pdf