Hello, I'm currently in the search for a method that allows me to downgrade from 4.2 to 4.0. But it doesn't seem to be possible without pre-saved SHSH blobs. Seeing as I didn't collect these blobs at the time, I struggle to find a method around this.

Can anyone confirm that the only way to downgrade is by having saved the SHSH-blobs?

If not, would a custom fw of 4.0 work? I'm getting all these 1600-errors. Would iReb fix this for me?

Thanks in advance

The iPod touch 2G doesn't validate SHSH blobs.
You should be able to use pwned DFU mode with an original IPSW to bypass the iTunes checks and downgrade to 4.0

Don't trust iREB - it mostly just hides the real problem.