Want help before I open the box...

My brother has a modest system with
Mod Squad Prizm2 CD-player,
Cambridge A300 integrated amp,
Vandersteen 1c speakers
and RS wires all arround.

The right channel of the amplifier sounds much more louder than the left and the balance control turned approximately on 8:30 to justify the channels.

If I play with volume control somehow after the small click-pop from the speaker the volume justifies and sounds again balanced.

I tested CD-player with my headphones and it seems to work OK.

What should I examine in the amplifier in order to detect the problem?

As far as I know this integrated has passive input stage and I believe that I have to look-up in the power amplifier part.