That you heard noise with only 30db of gain says noise is still an issue. To know if it’s from the recorder or environment, using an analog preamp in front of the recorder and see if it gets louder. Recording silence in the quietest area you can find with varying levels of gain from -10 to 60db would be an interesting test.
I suppose what I am testing is the standalone system - the demonstrable system noise produced from one end (built in mics) to the other (digits). To do that in such a way as there would be zero chance of the mics actually detecting any sound in the 'silent' environment is not simple (without perhaps a high tech soundproof room). And that's what is needed to be sure that any noise heard is system noise, not environment noise. As the gain and thus the recorded noise is fixed in the 32 bit float system, there is nothing to adjust. The recorder just assumes it's confronted by a rock band and it's been configured appropriately by the makers. What surprised me when running my test was that despite that configuration, the reverberation of the originally inaudible bird songs (inaudible by me on location) was clearly audible when 30dB of normalisation was applied. Not just the bird calls, but the reverb thereof. That sound must be close to the quietest sound one could normally encounter in life.
That scenario would, or should, be improved by connecting external low noise mics and preamps etc, indeed. But it would be interesting to demonstrate that even using inexpensive built in mics, noise is not a practical problem in most circumstances. Not all, of course.
I will see if I can devise another test... put the recorder between two mattresses in a countryside bedroom at 3am...
As for using noise reduction, I did actually also have the recorder set up with its built in AI NR being applied to another pair of tracks. That did drop the background noise (system noise or distant environmental noise, whichever) by 12dB, but it did subtly mess with the birdsong when normalised. How well it might work when recording music in an air-conditioning compromised performance space (for instance) I don't know.