What would cause the sound to be very low and the bass mod not to work on Bose Acoustimass 10 Series II home theater speakers hooked up to a Sony STR-DE698 home theater receiver?

Be sure that all speaker connections are the proper polarity. This means that the red wire goes to the '+' side of all connections. If your wires are not color coded, inspect them carefully for a small ridge running along one of the wires, or some other means of telling the wires apart. Check polarity on all connections. It's also common for Home Theater receivers to be shipped with the subwoofer output to be switched on-(rolling off the 5 surround channels to divert that bass to the sub output.)So go into the setup menu and tell it no sub-unless of course you have one.

EDIT: You have to connect the speakers to the bass module, and the bass module to the receiver, otherwise it will not work.