Just to clarify, since I think I was the only one mentioning RS transformers in this thread. I am not really recommending them. I'd recommend the AT transformers that I used for awhile. But I only picked up the RS ones in a pinch after losing the AT ones. Ran the RS ones only a couple times before I switched to a Beyer MV100 preamp.
I would however, as I mentioned before, use the RS transformers before I used a pair of transformers that only provided a signal in the 1000Hz-10,000Hz range (assuming that information is correct).
Jon -- thanks for all your input on this. I'm not following you in regards to your statements on transformers and the SBM1. If you are better off stepping up the signal amplitude earlier in the chain, wouldn't that argue for using transformers rather than attenuators? Also, the unmodified SBM1 uses a 4580 op-amp based gain stage for the mic pre input and another 4580 gain stage for the line input. So a transformer-boosted signal going line into the SBM1 would go through the transformer gain stage and then the single 4580 gain stage in the SBM1. If you went the attenuator> mic-in route, you'd be going mics> attenuator> 4580 gain stage> 4580 gain stage. So by using the transformers, you skip one 4580-based gain stage, opting instead for the line transformers. I have no idea whether line transformers would sound better than a 4580 op amp, but it seems that if they do there might be merit to going with the line transformers rather than going through an attenuator and then mic in. I'm curious why you'd always prefer the attenuator > mic-in route.