I know you said you sourced all the required SQL updates, but those symptoms are typical of missing one.
There should be some SQL related error messages in one of the zone logs in your logs folder.
From looking at the post rev 1900 SQL updates that would affect mob spawns, missing one or both of these seems the most likely candidate:
http://code.google.com/p/projecteqem...efinitions.sql
http://code.google.com/p/projecteqem...944_spawn2.sql