Check the fuse rating on the back of your amp. It’s probably a lot less than 20A. :) I don’t have access to the manual but I saw an image which specified 1500W on the back. This should be around 12 Amps. A 15A circuit is generally rated to 1800W, a 20A circuit for 2400, so definitely a good thing you are using a 20A circuit!
Any decent conditioner should also have a built in breaker, so in the unlikely event you overload it the breaker should trip quickly.
Generally speaking, there’s not a lot of difference between a 20A outlet and 15A outlet. They both have to be built for 20A due to the NEC allowing a mix of 20A and 15A plugs on a 20A circuit.
That’s not to say the wiring in the wall doesn’t need to match the breaker in your panel, it absolutely does. :)
The thing to consider for the long term is how much of those 1500 watts do you actually use while listening? The more of that you use the more likely you can make your line sag. To find out use a voltage meter like this one. Leave it plugged in and pay attention as much to the N-E as the overall voltage. This will also help you understand how much voltage variance you have due to seasonal/major appliance use and how much is due to your gear.