Yeah, its wierd - if you look, there's a number of threads about it - some people report needing to attenuate, and some people don't. This is also the case with a battery box -> AD-20 for some people - some report needing an attenuator (I did!) others don't. I haven't been able to figure out what it is, but my suspicion is the SoundPro's bboxes.
I'd just hate to see someone purchase and be screwed. But I still think the ModSBM-1 is a way better sounding item than an AD-20. Especially and AD-20 for $529. Considering that a used AD-20 goes for around $150 - $200, is that mod worth $350, since they can't be found used very easily? That's something the purchaser needs to consider, IMO.
My personal experience was that I needed attenuators w/ all of these rigs:
DPA4061 -> SP-SPSB-1 -> AD-20
SPMics -> SP-SPSB-3 -> AD-20
AT853Rx -> PS-2 -> AD-20
I guess its possible that my AD-20 was whack, as that's the only commonality in those rigs. But, if you do need attenuators, you can't attenuate that thing.