you're almost guaranteed to overload using an external preamp. Heck, even some mics w/ a battery box could overload the input w/ that rating. That's why attenuator cables are mandatory going sbd > mic in.
Maybe do it simply to test at home and rule out something wrong in the line input signal path, but I think that's an unlikely root cause.
Could help in troubleshooting.
What you say above excluding the part in bold is true- you can overload the mic input with a hot source, but as long as the line-in record gain is keep at 67 or above (and the recorder is set to low gain) there is no fear of overloading the input stage as long as the meters on the DR2d are below clipping, regardless of the source, external preamp or sbd or whatever.
My regular DR2d setup is 4 x 4060 (pretty sensitive mics at 20mV/Pa) > 2 x CA-UGLY > into both mic AND line inputs. The UGLYs use the same circuit as the 9100 AFAIK. Admittedly I use very little if any gain increase on the UGLYs for most things, and would need attenuation for really loud stuff which I don't record with that rig. But my setup with those external preamps works just fine for non-deafening music and the metering is accurate and trustworthy. With less sensitive mics like the CA-11s, which are significantly less sensitive than 4060s although I don't know exactly by how much, I could record far louder material.. which I would certainly would need external attenuators for if using the 4060s.