The Bose units do not offer true subwoofers, even the Bose models that do include separate LFE/subwoofer connections in additional to the F/R/C speaker connections. That's why you can most of the time do better (bass-wise) purchasing even a bargain basement sub instead. It's more noticeable outside of the stores in a typical HT or AV space.
The 600 doesn't even connect to the subwoofer/pre out either, which may be why on the pio, it takes some futzing to get halfway decent bass. The Pio receiver specialists @ AVS might be able to help.