Yeah, that's what I was getting at - suitability for use with an external preamp won't necessarily correlate with the complexity of the circuit path.
Yes, noise/distortion is a better, more appropriate measure.
The most fundamental/practical thing is making sure the recorder's line-in is capable of accepting the output level from the external preamp as typically used in your recording scenarios without overload. In other words, everything should work even if the external preamp is capable of producing higher output levels than the recorder's line in is capable of accepting, as long as the actual output from the preamp does not exceed the maximum input capability of the recorder. That situation is not at all uncommon.
For example: The first small digital recorder I purchased was an Edirol R09. Depending on what microphones I was using I would either run a Church Audio preamp in front of it for miniature microphones, or a Grace V3 preamp when I needed to provide phantom power. The line-input of the R09 can accept a maximum signal level of 0dBu, while the V3 is capable of outputs level of up to 24dBu. That meant, given the material I was recording and the microphones I was using, I never turned the gain of the V3 up very high.
With the R09s meters peaking where I wanted them, the meters on the V3 weren't moving that much. But both were operating in their comfortable a Everything worked fine.
Same would apply today if I were to run the V3 in front of the F8, although I never have. The F8 is spec'd to accept a line-in level signal of up to +4dB, so I could use a bit more gain on the V3 than I could with the R-09, but would still need to be careful not to overdrive the F8 inputs with too hot a signal.
The same would apply if I were running an F8N-Pro and recording in 32bit float, because the maximum input level the line-input is able to accept remains unchanged from F8 through F8N-Pro:+4dBu.
