I've lived with the Eclipse for over a year so my impressions on the improvement are over quite old, however I recall having two distinct thoughts.
First that the improvement was more significant from Signature to Eclipse than it was from stock to Signature. The second was that the the music had more nuance, which gave me more emotional involvement in the listening.
In speaking with Aesthetix they recommended doing the Atlas prior to the Calypso and Rhea. It was their instruction that the amp would yield the biggest benefit of the three. I have had the Rhea upgraded to Eclipse as well and the Atlas was more significant. My Calypso is still at Signature level.