The IO sounds beautiful and a considerable improvement, we only have one other source (CD) is the addition of the Callisto with 2 x power supplies really any better than this?
Yes, In fact a Callisto with single power supply added to the Io will boost your performance. I have personally tested the Io with several preamps and the improved performance is directly related to the quality of the preamp added.
I must say the Audio Research Ref 2 with NOS tubes was a very good upgrade although not as grand as a Callisto. I have not heard the ARC Ref 3 in my system with an Io but would like to.
Congratulations on your upgrade, do you know what tubes are in your Io, original or premium NOS?