1) I hope you switched the amp to Speaker A+B when you connected sub to B. Sometimes we can overlook these things.
Yes, I switched to A+B but felt bass became weaker than before or maybe my ears made this up.
2) only one rca will send only one channel's bass to the sub. That's very wrong for stereo. Y-cable is the correct way if using pre outs. Remember integrated is not sending a summed LFE like an avr.
Just read out your other reply and now I also think its not possible to sum up the L and R preouts. Only option remaining is to add a new sub to R preout. But I dont think I need that much bass. I look only to fill up music, so subtle bass is needed, maybe will revert back to high level outputs from terminal B to sub.
Against common opinion, I will advise you to once place the sub in the middle of two speakers out in the room (away from walls). See how it goes.
I can try this but my wife can kill me if she finds subwoofer in the middle of room.