I don't think the passive bi-amping is going to gain anything over and above what the amp can do with those speakers. Maybe work on getting the larger sub(s) sooner than later. Still also depends how low/loud you want to go overall. In any case the XLS215 for low bass isn't going to do as well as a good sub, seeing how quickly it drops off under 38hz (-3dB point, but the -10dB point is 36hz).