Bro, you may want to go to Autoimage Enterprise @ Amtech Building 159 Sin Ming Road. I just went there to troubleshoot a very complicated problem I have with my system. They were fully consultative, non pushy and were happy to troubleshoot and diagnose my problem. They even explained step by step how they eliminate the items. Service level was top notch. Best I've ever experienced for any type of service industry. They don't make you feel stupid or just say that it is because of your head unit or speakers or amp. They show you the problem and leave you to decide. Mind you, I change 4 speakers and subwoofer with 1 place and they were not even genuine in wanting to help to solve my problem.
For my problem, trust me, I went to a number of shops. Some are unhappy to entertain because feel it is too troublesome, some just say that it is either your HU or amp without even wanting to diagnose. After a series of elimination exercise, found that it is the amp that is the problem. Changed it and tuned it to the extent it was perfect to my ears. I just wonder why they are not mentioned here more.