TBH,
Assuming it’s a reputable place you’ve taken it to, then it sounds like they’ve done the ‘right’ thing in so far as they’ve serviced the amp as well as repairing it. They’ve probably saved you from having to return it to them again six months later (perhaps accompanied by a shitogram going “I only sent it back to you a few months ago, you’re rubbish, etc”).
However, they shouldn’t be undertaking work without your consent, and I’d be annoyed if it were me. I think legally though, your only real course of action is to tell them to undo the unauthorised work and put it back as it was, or to “pay under duress” and then persue them in small claims. I think.
Ultimately it’s up to you to decide whether you think they’ve been unreasonable and act accordingly. I think if it were me, rather than “complain” I’d query it with them, “I didn’t ask you to do this and it’s a lot more expensive than I was expecting, what gives?” sort of thing, and see what they say.
Did you ask them for an estimate at any point?