How to increase audio output/Sony Amplifier TA-AV601?

I have this amplifier that has the capability to hook up numerous speakers. I tested it with a Sony Tuner that is fully functional and connected equally healthy speakers (2 ea.). Yet, when turning up the volume to a normal level, the amplifier doesn't respond to the dial. It stays at a level that one might hear in a doctor's office waiting room. It doesn't matter whether the tuner is replaced by a tv for testing, the low volume level governs the test result as a 'fail' right from the beginning. The fuse under the cap looks fine. I have seen this amplifier operate up to 8 speakers without even a struggle and the performance was staggering. It was a dvd and I felt I was at a cinema. Tons of high end/chair shaking bass. The Serial Number for unit is 806203. Lastly, I thank you in advance for any guidance on this.