OK chaps, brains in gear... I've been doing some measurements on my 'stock' R-44 (24 bit files). All figures are average RMS below full scale in dB.
Firstly I investigated whether, when you go from click to click on the clicky level knob (the preamp knob), what difference there is from step to step.
To do this I simply recorded a steady tone at each step, going up from the lowest gain, and the results are as follows.
Average RMS
Clicky 24 bit tones
-78.07
-71.82 = 6.25
-65.68 = 6.14
-59.42 = 6.26
-47.09 = 12.33
-40.88 = 6.21
-34.8 = 6.08
-28.66 = 6.14
-22.38 = 6.28
-16.74 = 5.64
-10.71 = 6.03
The figure after "=" is the difference from the figure before. So from lowest setting to next one up, gain goes up by 6.25dB. Then by 6.14, etc etc.
Note that from the postion "-14" on the R-44 display of gain to the position "-20" the gain increase is actually 12.33dB not 6dB. The rest are ballpark 6dB jumps.
Now let's see what we can tell about the smooth control, which I suspect is just digital gain. At its lowest setting, when recording a steady tone at a fixed clicky gain setting, you get digital silence (all bits zero). As you increase the setting to some arbitrary points you get these pairs of signal and noise results (noise measured with a 150 ohm resistor plugged in) -
Smooth 24 bit tones & noise
-39.86 signal
-106.58 noise
-23.85 = 16.01
-90.48 = 16.1
-17.03 = 6.82
-83.67 = 6.81
-13.77 = 3.26
-80.39 = 3.28
-11.07 = 2.7
-77.78 = 2.61
-8.95 = 2.12
-75.63 = 2.15
Again, the figures after the "=" sign are the change from the previous values, and you can see that signal and noise pretty much increase by the same amount as you raise the knob from its lowest setting to its highest setting (as represented by the last pair of figures). This to me is still the kind of behaviour you would expect from digital amplification - signal and noise increase by the same amount as you go up. Put it this way - it shows that there is no particular point in amplifying the signal this way vs doing it in the DAW as you don't get an improvement in signal to noise ratio either way. Analog gain change vs noise change tends to not be linear (see next lot of figures).
Lastly, below are some figures just for the noise floor at each position of the clicky gain knob measured across 150 ohms.
Noise 150 ohm
-107.42
-106.24 = 1.18
-104.42 = 1.82
-101.68 = 2.74
-106.65 = -4.97
-104.26 = 2.39
-100.42 = 3.84
-95.35 = 5.07
-89.48 = 5.87
-83.85 = 5.68
-77.84 = 6.01
This shows the noise floor being at -107.42dB with the knob at its lowest setting. When you increase the gain by one click from the bottom (gain increase of 6.25dB, see first table of figures) you get a noise increase of only 1.18dB (to -106.24dB). So that's a better signal to noise ratio if you don't get clipping. And you can read off the figures for other clicks of the knob.
Note that when you go from the position labelled "-14" to that labelled "-20" on the display, you are getting a gain increase of 12.33dB but a fall of noise floor of 4.97dB, so there is a very significant improvement of signal to noise ratio at that point. I would interpret that as being the setting from which which Edirol assume you will be using mics rather than line inputs. At each point above that, you get more gain increase vs noise increase with each click of the knob, though the differences become small for the last four settings. One could argue that recording with the clicky knob higher than the setting labelled "-38" on the display is not really worthwhile in terms of improved signal to noise ratio - you might as well raise the level in your DAW if need be, as that too will raise the noise and the signal equally, but with the knowledge that you won't get unforseen clipping.
Bottom line - have the clicky knob at "-38" or below (because you might as well use your DAW to amplify above that figure) and avoid going as low as "-14" when using mics as the loss of gain when you go down one more click is 12dB, while the noise goes up 4.97dB at that point. And you might as well keep the smooth control at noon as if it isn't a digital gain control, it certainly seems to act like one.
Whew! And goodnight.