Does this experiment lead us to conclude that the MOTU's mic pre's are not very good sounding mic pre's?

 
The onboard MOTU preamps will not match up to the Millennia preamp.
The Millennia is a world-class piece.
The same is true for all included onboard preamps... on all audio interfaces.   
I've done some location recording using the onboard MOTU preamps... and they do a fine job.
Not exquisite... but pretty good.  In short, they're reasonably quiet... mid-level preamps.

I use a rack of fancy preamps and go into my MOTU 896HDs with the +4dBu switch on so I bypass some of the preamp. If you asked me I think this gives me a lot more detail in the lower frequencies.

I have often wondered what it would sound like if I also had a rack of great converters.

If you want to use a converter like the Lucid to bring in 2 channels you can expect to get great sound.

If you fill a rack with 8 to 12 pairs of two channel converters like that Lucid... you have a clocking nightmare... the more you learn about that the more complicated it will seem.

So, If you want to bring in 16 channels etc... there are very few straight forward choices.

I've been considering a Lynx Aurora 16... and really want that kind of quality in a single 24 channel form factor... but just don't know how much better it will be.

For now, my MOTU stuff sounds better than anything I ever used in the past.

best regards,
mike