It is often the case that repair places will charge a fixed rate for the diagnosis, such as the one hour time, even though it might take much longer. The fixed rate protects the customer; the customer is not charged for several hours only to find out the item is unrepairable. That $140 does not cover the expense and time involved in unboxing, potentially boxing up and shipping the item if a repair is not attempted, so there is some incentive for the shop to investigate even this exceeds that one hour diagnostic time, with the hope of recouping the extra cost via the repair job. If the repair is approved, I would expect that extra time diagnosing to be added to the total repair time.
As to the total of 14 hours, this does seem to be a lot of time, but, you seem to have the experience to have made an assessment at the time an estimate was provided. If you had told them this was too much and you would just pay for the assessment and you want the amps back, they might have found a way to lower the price. The $140 to check out your amp and do nothing else would clearly be a loss to the shop. I don't like to haggle, so I would have probably just accepted the estimate, but, this is clearly an issue with you; the problem is that you should have acted before, not after, the repair.
As to the total of 14 hours, this does seem to be a lot of time, but, you seem to have the experience to have made an assessment at the time an estimate was provided. If you had told them this was too much and you would just pay for the assessment and you want the amps back, they might have found a way to lower the price. The $140 to check out your amp and do nothing else would clearly be a loss to the shop. I don't like to haggle, so I would have probably just accepted the estimate, but, this is clearly an issue with you; the problem is that you should have acted before, not after, the repair.