It was the low-fare avaibility to TXL on 5/1 (arrive 5/2) that dried up.

There are still TXL runs available in the low $400's on other dates, but the TXL-AMS flights on 5/2 were the only ones that offered a cheap routing to AMS.

Thanks for the pointers. I'll be checking these fares often in case something becomes available again. Already have 3 runs to Germany this spring (2 TXL, 1 HAM), hoping to end up elsewhere for variety. AMS is one of those places where we've yet to leave the airport.