I guess I don't understand how running external attenuators is going to sound better than using the attenuation of the bit mapper.
It'll sound better since the output of the V3 is way too hot for the SBM1.
Really, I don't understand how people say they run this combo without attenuators. A local taper here swore he'd do that without issue (I think he has since recanted), so I tried with my gear back when I had an SBM1. The recording came out totally fuct. But he kept swearing he did it with no problems, which made me wonder if my SBM1 was bad.
Then I started modding SBM1's, and would always test each one before working on it, to make sure it was working properly. As part of this testing, I decided to also test out the V3>SBM1 issue, with the V3 running at -2dbFS to 0dbFS (ie, hot). Not a single one of the SBM1's I tested could handle this signal coming out of the V3. Always clipped/distorted.
Then of course, I tried the more obvious -- rather than experimenting, I just read the manuals. At 0dbFS output, the V3 puts out a +24dbu signal on its analog outs, +21dbu if run unbalanced. The SBM1 manual says the SBM1 can take a maximum signal level of -3.8dbV (about -1.5dbu). So the V3 run hot puts out a signal that is 22dbu higher than be accepted by the SBM1. Give it this level of signal and the SBM1 will totally clip/distort/brickwall. The level attenuator on the SBM1 is post-gain, so using this will not help at all -- you're still feeding it far too hot a signal.
I got some resistors to make attenuator cables for the MT, but never got around to doing it. I'll have to check the values, but I should be able to make attenuator cables for use with the SBM1. No problem building them into regular XLR connectors, but I'm not sure about building them into right-angle XLRs.