Neither o fthe Wadias I have owned have had a 100 volume setting - just 99, so I can't comment on Aoliviero's experience. In my systemthere isn't a significant difference between 98 and 99, or any close volume settings. If you have a 100, though, definitely try it.
If you want to do an A/B comparison of the sound with and without a pre-amp, first set the volume of the Wadia direct into the amps at the desired listening level. It should be over 67, as Wadia says. If not, open the unit and set the dip switches. Then connect your pre-amp and adjust the pre-amp until the volume is the same. Your pre-amp is now set at unity gain, meaning it doesn't change the volume. Once set up, this is the easiest way to swap back and forth. To change the volume, use the volume control on the Wadia. That way, when you swap the preamp in and out, the volume will be the same, whether the pre-amp is in the chain or not. This will give you a good idea of what your pre-amp is doing. I suppose the best way to run the test, though, would be to play the Wadia at full volume when connected to the pre-amp, and adjust volume with the pre, as you would do in real life. The problem with this is that most pre-amps are very difficult to get repeatable volume settings with, so you are likely to be listening slightly louder or softer, which will skew your results.
My experience with preamps is that they degrade resolution, but provide a richer sound. Much is made fo the degradation of resolution when using the Wadia digital volume control, but I don't find this to be the case. Plugging in a pre-amp creates a much more noticable loss of resolution than dropping the volume from 99 to 70 (which you can play with either by having the Wadia plugged into a pre-amp and turning down the Wadia then turning up the preamp, or by listening without a pre amp, then plugging the Wadia into a pre-amp, setting its volume to 99 and adjusting the preamp's volume).
If you want to do an A/B comparison of the sound with and without a pre-amp, first set the volume of the Wadia direct into the amps at the desired listening level. It should be over 67, as Wadia says. If not, open the unit and set the dip switches. Then connect your pre-amp and adjust the pre-amp until the volume is the same. Your pre-amp is now set at unity gain, meaning it doesn't change the volume. Once set up, this is the easiest way to swap back and forth. To change the volume, use the volume control on the Wadia. That way, when you swap the preamp in and out, the volume will be the same, whether the pre-amp is in the chain or not. This will give you a good idea of what your pre-amp is doing. I suppose the best way to run the test, though, would be to play the Wadia at full volume when connected to the pre-amp, and adjust volume with the pre, as you would do in real life. The problem with this is that most pre-amps are very difficult to get repeatable volume settings with, so you are likely to be listening slightly louder or softer, which will skew your results.
My experience with preamps is that they degrade resolution, but provide a richer sound. Much is made fo the degradation of resolution when using the Wadia digital volume control, but I don't find this to be the case. Plugging in a pre-amp creates a much more noticable loss of resolution than dropping the volume from 99 to 70 (which you can play with either by having the Wadia plugged into a pre-amp and turning down the Wadia then turning up the preamp, or by listening without a pre amp, then plugging the Wadia into a pre-amp, setting its volume to 99 and adjusting the preamp's volume).