Since the cancelled flight was AMS-PRG wouldn't EC261 come into play?
I thought about that, but does it really help? I think that would be 250EU which is only a few bucks more and if OP goes thta way I doubt DL would do any more.
So unless I have this wrong, OP would be fighting an EU 261 claim for something like $30 (over the DL offer) vs just asking DL for more and seeing what happens.
EDIT: I missed that DL already played the EU-261 card.