Sure, I'll do my best.
First off, I did not make any part upgrades and stuck to the original parts list.
From what I can tell, the Castanet plays music truthfully and balanced, never seeming too bright or overwhelmed by the lower end. The highs are articulate but not harsh and the low end seems full-bodied and accurate. The imaging is excellent as well - instruments are clearly separated, never bleeding into one another, and there is a huge sense of air and space.
The noise floor is almost non-existent. When hooked up at the end of my vinyl setup, there is a small amount of hiss that I suspect is coming from Piccolo/Cornet2 configuration, and is not audible once music starts to play. When hooked up to a digital source, I can not hear any noise whatsoever - even with the volume way up. In fact there was a few times I got startled when I hit play because I didn't check to see where the volume knob was set at.
But simply put, the Castanet sounds great to my ears and I would highly recommend it.
I can't comment on how the Castanet will work with an electric guitar, but I imagine any line source would sound fantastic.
Scott.