cruise01

I have a 01 GS300 w/ Mark Levison audio system/Navigation. PROBLEM: front speakers are making a popping/static noise - whenever the car is started it last for 2-3 minutes. Thereafter either there is no sound at all for days at a time or the radio will play normally. I have taken it to several audio dealers and left with false hope of it being repaired. Some of the feedback I received was that the problem in the speakers, radio and or amp. Based on the research I have done - I believe it is the amp. The other problem I am faced with as of late is finding someone who will install an aftermarket amp. I recently had a tech tell me that if he did the repair it would affect the navigation system. At this point I am wondering if I just need to find a comparable Lexus part and just replace it. Does anyone who has had this problem know where I can get a replacement part?