It appears the MA6500 is stable to 2 ohms, but still best make sure there won't be a problem.
What is the impedance of living room speakers, and does the .1 subwoofer have it's own power?
Here is an example to show you a proper way to install a single in-ceiling subwoofer, assuming you want it powered by the MA6500.
OEM Systems
Use the dual voice coil model of the woofer and 2 crossovers, one for each channel, then wire using the Crossover Option 2 diagram on page 16 of the manual.
All of the above can also be accomplished by using a SpeakerCraft 8.2 BAS. This in-ceiling woofer is a dual voice coil and has the crossovers mounted on the back. It also has a phase switch and the crossover is adjustable.