when running line-in, the SBM-1 (modded or stock) does not exhibit the same brickwalling problems as the Sony DAT decks. Doug Oade told me this in an email, and I confirmed it at several shows. I never had a problem with brickwalling, and I've run the SBM-1 down at 2 or 3, and maxed out at 10.
the mod SBM-1 (Doug's line-in mod) disables the mic inputs, and therefore the unit will not provide much gain. a stock unit should give you enough gain for just about anything.