I sympathize, but you would have to prove negligence on their part, to prevail. Expensive to prove in court - requires not just an attorney, but also an expert witness is usually required to prove the applicable standard of care. Not at all clear from your version of the facts that they were negligent. Especially where, as Brf points out, these amps are well known to be trouble-prone, at best.
Many shops have a disclaimer posted, or that customers must sign, that absolves them from, or limits, their potential liability
Lloyd the Lawyer