My reading: It's not eurobonds which are unconstitutional, but the ESM or similar Euro mechanisms taken without specific Bundestag approval.

I can't cite anyone specific. I did ask my tax/financial advisors, who are part of the Mittelstand team, who advise many German companies. They specifically said eurobonds are not unconstitutional, unless they don't carry specific approval. I believe what's unclear is the nature of the mechanisms used to administer Eurobonds or equivalent. But that is something the Bundestag could fix.

(I'm very pressed for time today, tough to research further)

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin