3M Scotch makes several different double sided mounting tape products. The one you need is the extreme heavy duty version of the grey with red backing outdoor rated tape. I've found it in various hardware stores, auto parts stores, and places like Home Depot. If you prep the surfaces properly and apply the tape properly (apply to one part first with even pressure for at least thirty seconds, remove the red backing material and then press that surface against the second part for at least 30 seconds, followed by several hours of letting the bond strengthen), you might be surprised how well it holds and how hard it will be to break it loose in the future. I'd recommend you lay the bag down so the weight of the amp is pressing the tape against the bag and not hanging from it, thus keeping pressure on both sides of the tape while the bond strengthens. Just make sure to use a large enough amount of tape, the weight rating is per square inch. Hanging something heavy from just a short length of 1 inch wide tape probably won't hold up long.
Jerry