Is there a difference in performance between my original LJB4170BA and a replacement LNC4170AA ??? Footprints are identical, but new one also provides no sound.

 

Looking that amplifier up it appears to be for the XJ8 rather than the XK8, so it may well not be compatible. You could compare the wiring diagrams of your car and the XJ8 and see if they match, but even then the amps might not be compatible. I assume you have checked the fuse for the amp and made sure you have power and ground at the amplifier?

 

^^ +1
I've quickly checked the guides, and the connector pinouts appear to match, but the amps have internal loudspeaker crossovers tailored to the model (hence the different part #s) so the swop might not sound so good.

Fuse #9 (10A) in the trunk fusebox feeds the amp.