Unfortunately, your Cary CAI-1 lacks preamp line-level outputs, so you have only one choice.
Connect the second set of speaker output terminals of the Cary CAI-1 to a switch (DPDT on-off) and that switch to the the sub's speaker-level input terminals. Leave the sub out from the AVR connected as-is.
To use this setup for HT, make sure the switch is open (sub not connected).
To use this setup for 2 channel, make sure the AVR is OFF and then close the switch.
The problem is that the level/crossover in the sub must be the same for both operations and that will never be ideal for both. There is no reasonable solution for this but I cannot think of any other way with the equipment you have.
Connect the second set of speaker output terminals of the Cary CAI-1 to a switch (DPDT on-off) and that switch to the the sub's speaker-level input terminals. Leave the sub out from the AVR connected as-is.
To use this setup for HT, make sure the switch is open (sub not connected).
To use this setup for 2 channel, make sure the AVR is OFF and then close the switch.
The problem is that the level/crossover in the sub must be the same for both operations and that will never be ideal for both. There is no reasonable solution for this but I cannot think of any other way with the equipment you have.