Question... I have an ISA One mic pre and I'm trying to decide... Which inputs on the Babyface Pro would be best for the line out signal coming from the preamp... the XLR (mic/line) or the 1/4" (instrument/line)?

I don't have that unit, but I run my ISA One preamp via a TRS plug to a Mic input on my MOTU 896Mk3 Hybrid.  The inputs have both XLR and TRS inputs and using the TRS input bypasses the MOTU mic preamp.  Thus in your case, I would use the XLR Mic input.

either works well,
as long as the correct cable is used.
trs or xlr.
 
the preamps of the BFP are real clean, 
so if you are conservative with the input,
you can drive all the color you want with the ISA, and keep the converter side clean

The problems is the XLR ins on the Babyface pro are expecting mic level input, so there is a possibility of overload.  The 1/4 in inputs are unbalanced, but they are the ones you should use, just keep your cables short.

For an input from an external pre-amp I'd use a line level one.

Pre-amps generally output at line level and are likely to overload another downstream mic pre. The RME pres are very quiet, but they still have a little noise which will be added to the other pre's noise floor, while the line inputs don't apply as much amoplifiation to the signal and so are quieter.

Whichever way you go you'll need to set up the gain staging in Totalmix of course.

rumleymusic
The problems is the XLR ins on the Babyface pro are expecting mic level input, so there is a possibility of overload.  The 1/4 in inputs are unbalanced, but they are the ones you should use, just keep your cables short.

tlw
For an input from an external pre-amp I'd use a line level one.
Whichever way you go you'll need to set up the gain staging in Totalmix of course.

1/4" it is!

Gain staging... that's a thing I'll definitely have to work out.
 
Thanks, guys.

rumleymusic
The problems is the XLR ins on the Babyface pro are expecting mic level input

not this unit, very flexible

First of all line input levels can be fed into the Babyface XLR inputs. The gain of the mic pre can be turned right down and they suggest you use the pad as well. In that mode a +4 dBu signal could easily and comfortably fed into the Babyface. However you are still running a Mic Pre into a Mic Pre. Which will be fine in most cases. But overall it might be better to only involve one Mic Pre at a time. After all you have bought the ISA Mic Pre for a reason. Because you must feel it is better then the internal Mic Pres of the Babyface. And it probably is (although in real world conditions I doubt you would hear a radical difference. The choice of microphone and placement will far outweigh the differences in the Mic Pres sound. e.g. I bet I could get great sound in the Babyface alone. But you do have the ISA so there is a better way to connect it then)
 
Only the line inputs on the side of the Babyface are a true line input and they don't have any Mic Pre involved. 
 
I would use XLR to either TS or TRS and keep the cables short i.e. have the two things close together. The line inputs of the Babyface can be set for either -10 or +4 and I would set that to +4. I would not alter any gain settings in Total Mix either.  Just change the input sensitivity. 

actually,
the preamps of the babyface pro are exceptionally clean.
 
so, if you WANT color,
you simply add a colorful preamp, watch the output of that preamp, into the BFP at proper levels, and all is good.
 

There are really three sounds you are dealing with:
 
1  Mic direct into Babyface Mic Pres only. (Sound is BFP Mic pre only)
2  Mic into ISA and then into Line inputs of BFP (Sound is ISA is Pre only)
3  Mic into ISA and then into BabyFace Mic XLR inputs (Sound is two Mic Pres in series)
 
Sound for versions 2 and 3 will probably be similar due the presence of the input transformer of the ISA in both these cases.
 
Gain or distortion overload etc is not an issue with any of these setups, it is a matter of sound. The smart thing to do is try all of these on exactly the same source and listen and use your ears. Make sure the final playback levels are identical for all three. 
 
There is a nice input transformer on the ISA so it is possible you will like that sound. But remember transformers introduce a form of distortion no matter how you look at it.  Judging by the size of the Babyface I doubt there are any input transformers at all hence it might be a slightly more pure and transparent sound. So for some that is actually a superior sound.