With only one source, Don's method works, the other preamp output won't be filtered.
With two sources you are using tape loop on preamp, both ouputs are filtered by BAM. using this configuration the BAM essentially remains between the source and preamp. I can't think of a way to maintain BAM between source and preamp and add subwoofer without it being filtered by BAM on preamp outputs. The BAM would have to go between the preamp and amp.
Yes, with two BAMs you could connect each source to it's own BAM, BAM outs into source ins on preamp, preamp outs won't be filtered. I'm also wondering if you could add a second set of RCA ins and outs to an existing BAM. Both sources would go directly into BAM, out into preamp, no filtering at preamp outputs. I think I asked Bobby about this some years ago, can't recall his comments. You could also use a balanced BAM (has two complete circuit boards), one circuit board for each source, outs into preamp ins. Preamp outs no filtering.