Before making circuit modifications it would be a good approach to be extra careful to verify that the original circuit is operating properly and is configured according to the original design. Since the amp was a kit would you please post the model of the kit information? Someone will then be able to find a copy of the original kit schematic. If the kit had known defects then someone may know about them and the remedies.
Also important information is that you said "On testing with signal gen an scope sine wave is prefect." This is a clue that the problem may be elsewhere in your set up than the 6V6 Power Amp. We should consider that.