Alan you are right, probably a cracked logic board. I found this out today by connecting my speaker system to a HK avr130. No significant changes in sound output from the left surround with the 130.
I decided to return the 630, and buy a HK avr7200 because I (we) play music more often than watch movies. I will enjoy having the extra headroom the 7200 offers.
One thing I learned (think I am right) is that the total sound output using logic 7 is a lot less than straight stereo. So it can't hurt to have the extra power.
Intune
