Sounds to me like you have a blown amp, or preamp stage in the receiver. Depends on how much money you want to sink into it, you could get a service manual, and start there. In my experience, HTIB systems are rarely worth fixing, as the cost exceeds the price of a new unit VERY quickly. It sounds like you have a multitester, you can check the speaker lines for continuity, and any sort of voltage output to see if you have a damaged amplifier. If the Preamp/input, and Amp stages are on separate PCBs, you can also test for voltage across the two.