You have one sub out on your receiver so you run a y splitter into the two input of the Adcom, you then hook a pair of speaker cables to a stereo sub..sounds right to me. Passive sub needs an amp and amp need a signal. Something wrong with the Adcom? If you only run one amp channel to a stereo sub it will only output half as it has two voice coil setup and is the reason for two inputs.
Dave
Dave