Well, I was really just guessing the SBM1 would take +4dbu. But I just checked the manual in the archive section. According to the manual, the SBM1 can only take -3.8dbu (500mv) at the line in. The V3 puts out +19dbu on unbalanced analog out at 0dbFS. So it looks like to be safe, you'd need 25db attenuators (22.8db to be exact). Or run the V3 with peaks no more than -24dbFS, which doesn't sound like a good idea if you have patchers or are running a deck straight out of the V3.