Well I did some measurements of the iTube using my RME ADI-2 Pro. In 0dB gain buffer mode.
Upshot is it probably won't have much of a sound unless you drive it really hard. Given it's high input impedance it should be good as a buffer. May remove a bit of detail for those with the ear for it.
Frequency response 1.5V RMS in, very flat:
THD(&N) sweeps up to 6V RMS in (0dBFS bottom scale). SNR and THD good around 1-2V. 2nd harmonics dominates at high levels.
1kHz tone spectrum around 1.5V RMS in (ignore the 3V, gain seems about 0.5dB in 0dB gain mode: I am using RME unbalanced, you lose 6dB on the outputs but none on the inputs so -1dBFS out indicated translates to -7dBFS indicated on the input to the RME. Unweighted noise 92dB referenced to the input. I did vary and I have seen it up to 96dB at 1.5V in to the iTube.
Multitone around 1.5V peak in, not too bad, 15 bits resolution or so.
CCIF around 1.5V peak in. Similar to the THD but to be expected after looking at the multitone which is pretty flat:
If you are wondering if the digital antidote does much, here is impulse and step response without it on (through DA and DA "sharp" filters:
Here it is with the antidote:
Note that in normal use you wont get the AD ringing so the effect will be greater. It does reduce ringing a bit.
If I had my time again I would use a AD filter in the RME with little ringing so I could have focussed on the DA. The RME has various choices for both.
Plus it changes the top end frequency response a bit: