Brian makes a good point. I used a couple of the AT CP8201 line transformers (can be had at guitar center for under $20 each) to give me a bit of gain to work with and it was nice to work with the extended headroom. Since I use the Mod-SBM-1, the 1/4" was not available to me, but I agree with Brian, that may be the easiest way to feed the SBM-1.
Specific questions, feel free to PM me. I ran 391(3)s>B18s>AT-CP8201>Mod SBM-1>D100 for over a year, and may be able to help out.
Nick