By no means am I a lawyer, but imo they have no grounds at all for defamation. They could have been having a bad day when you used their services. That's just business. If they can take you to court for writing a bad review, then what of "word of mouth"?
I say fuck them!
Write another shit review, and include details about the cease and desist letter. Even inlude proof maybe, without revealing anyone's personal information. That'll teach 'em.
Can't claim defamation of character imo when it's the truth...