They're not going to re-fabricate a chassis for you. The amount of work that goes into fabrication is going to be far more costly than just giving you a new unit.
I'd say they owe you a new preamp, period.
Unless this is coming from the very top of the company (which I doubt), I have a feeling someone at a low level messed up and is trying to cover his or her tracks. I also have a feeling someone is getting fired.