dprincipato
Specs taken from HiFi Classic test: "The GFA-555 required an input of 0.13 volt for a 1-watt!! reference output" (Very very low gain)
The old war horse 555 is very low gain at only 130mV for a 1volt output. Which is: with a 2v source and a passive at full level, the 555, can only reach 16v!!! to the speakers
You either need a very high gain preamp, or seeing the Adcom is very old now you may want to get another amp, which most will have much higher gain than this old 555 more like 1.5v for full output (not 1v) and your source and passive will be fine then.
Cheers George