Power Amp Repair Question

It is fair to ask for return of your money. One month is plenty of time to diagnose the problem. They either lack technical ability or are just too busy. Either way, from your perspective no work was done and you have right to demand return. If they refuse call your credit card company to apply back charges or tell them you will file claim in Small Claim Court. Their position is unreasonable and perhaps you should warn other people?

They had a $100 estimate fee and I assumed I'd get an estimate with a diagnosis. After one month, the shop doesn't know what's wrong but says I lose the $100 if I want it back. I asked if they might know in 2 more weeks and they said probably not.
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If I were you I would not threaten them with any action until you have the amp back in your possession and off their property. Hopefully you have some type of a receipt that shows you paid them $100 to look at the amp and they would then follow-up with what was wrong with amp and what it would cost to repair it.

If so, and you paid the $100 using a credit card, then you can do a chargeback. If not I suggest you lick your wounds and move on, and be thankful you got the amp back, hopefully in the same condition you took it to them and not in a cardboard box in pieces.
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Forfeiting the $100.00 and getting the amp back might be considered money well spent considering the way they have treated you.