I often have run Sbd>SBM-1 with some XLR>RCA cables - you lose the balanced signal (which you do anyway when you run pre>SBM-1) but I've never had a problem with brickwalling with the SBM-1 and a XLR Sbd feed, while I definitely have trying to run the same feed straight into a Sony portable DAT. I always used the rule of thumb that if the SBM-1 had levels set to less then 4 then I would turn down the feed at the Sbd. I resolved all of this by lugging along my R500 to shows - I can run XLR straight into it, use SBM, and have a lot more confidence in that deck then I do in a sony portable. Of course it is a pain to lug that thing around but it's never failed me. I learned that trick from Bob Daniels (Hot Tuna), who has a rollaway with a R500 and a R300 in it. That amount of gear is probably not going to go over well unless you are in good with the venue and/or band but it is definitely bullet-proof. Of course once I got a taste of Sbd>AD2K+ @24/48>laptop, the 16bit DAT became a never-listened to back-up anyway.
Anyway, intersting discussion - I have an MP-2 and have never considered using it for anything but driving my KM140s.