I would call B&K tech support and ask them. You may want to be more specific about "no bass". My experience with the ST140, and ST1400 (I had both of them) is as Schipo says - they don't generate a lot of bass compared to many other amps.
B&K ST 140
Recently I took my B&K 140 amp out of the closet to test something in my system. I have not listened to it in years and had gone to back to tubes (this is not a tubes vs. ss debate). The B&K sounded as though it had no bottom end, no bass. Is it possible for the amp to work but not send all the signal? Maybe a fuse I don't know about?