I have experimented with my Wadia 850 running direct (adjusting the internal switches so that digital attenuation is minimal), and also through a number of preamps including Pass X1, Levinson 380S, ARC LS25, BelCanto Pre1, Art Audio, and others. Frequently the preamp would "modify" the sound in a pleasing direction, e.g., focusing vocals, improving bass definition and macrodynamics (the SS preamps), smoothing out the treble (the tube preamps), and so on. However, every time I came back to the direct Wadia, I was impressed with the "palpability" and realism of the sound, especially on difficult music reproduction like piano. So in the end I've just gone with direct. (The extra interconnect cable required for the preamp no doubt adds some flavor as well.)
Remember that Wadia artificially increases the bits to 21 in the DSP (I can't claim to really understand this), and that every 6dB of reduction on the display corresponds to a loss of one bit. Thus dropping to 70 still leaves you at 16 bits, and thus no net "loss" of information. Of course it is better to stay closer to 100.
When comparing Wadia to other CDPs with analog volume control, such as Res. Audio CD55, Levinson 390S, and Audio Aero Capitol 24/192 (all which I've heard in my system), the differences I hear are IMO largely due to other features of the design besides the output gain control. By the way, these are all great sounding CDP's, but they span the full range of the "organic to technical" spectrum of personal preference. For me, I like the immediacy and harmonic-correctness of the Wadia midrange, even if Wadia can be bettered in other aspects by some players. (I should mention that my 850 was modified by Great Northern Sound; this gives a nice improvement over the stock unit.)