Why do I have to turn the volume control on my 851A up past halfway to get the volume level I want. Is this normal?

The 851A volume control is slightly different to other Cambridge Audio amplifiers. The volume control is a completely linear scale so halfway is exactly 50% of the rated output. With amplifiers that use a more conventional potentiometer, halfway is near the maximum rated output. This can initially make people believe that the 851A is less powerful than it actually is. It is perfectly normal to use more than half of the volume control and this will allow you to achieve the level you want.