Difference between xrcd, xrcd2 & xrcd24???

[ahhian]

Pardon me for my ignorance, but what is the difference between the above 3 formats?

 

Which one produce the best sound?

 

I know its not a fair comparison, but between a sacd & xrcd, which is better? I'm talking about playing it via a normal red book cdp, not a dedicated sacd player?

 

Cheers