I have not heard the Callisto preamp at all, but I have a very strong opinion of it since it is from the USA. Through extensive listening sessions with both the Radio Shack and the Messenger preamp, In my very strong opinion, the Radio Shack is by far the finest preamp being made today. As good as the Messenger is, the Radio Shack makes it sound very tired and small. The Shack has the biggest soundstage of any preamp that I have never heard and the tonality is right on. It is extraodinarily dynamic and fast but retains a harmonic retrieval and richness that no other preamp that I have never heard can match.
In this case my advise is, please DO shoot the Messenger.
In this case my advise is, please DO shoot the Messenger.