Update:
I disconnected the Aiwa from the MSB Link III and connected my Sony DVD player to it. After trying both coax and optical connections the problem wasn't solved. I am, however, better able to characterize it. The sibilance is most noticeable in words that begin or end with "s" as opposed to having an "s" in the middle.
I also opened up the MSB Link III to ensure that the upsampling board hadn't been knocked loose. It wasn't. But I did discover something that was a little disappointing. The upsampling board plugs into two rows of pins in the middle of the "main" board. There is a tall capacitor under one corner of the board that prevents the sockets from sitting flush i.e. it isn't completely seated.
I disconnected the Aiwa from the MSB Link III and connected my Sony DVD player to it. After trying both coax and optical connections the problem wasn't solved. I am, however, better able to characterize it. The sibilance is most noticeable in words that begin or end with "s" as opposed to having an "s" in the middle.
I also opened up the MSB Link III to ensure that the upsampling board hadn't been knocked loose. It wasn't. But I did discover something that was a little disappointing. The upsampling board plugs into two rows of pins in the middle of the "main" board. There is a tall capacitor under one corner of the board that prevents the sockets from sitting flush i.e. it isn't completely seated.