Would you kindly expound on the type of layout problem which might result in this speaker buzz. Can a noisy toroid transmit buzz to loudspeakers etc
Generally a noisy power transformer will not cause noise in the audio path as well. But there can be AC line problems that can cause both noise in the amp and mechanical noise as well. Sometimes it can be confusing to sort them out.
The fact that one channel is noisier than the other is a clue. Without knowing any more about the amp I can't say more about layout, but that is what it suggests to me. IMO the amp should get checked out.