Help with DIY dedicated circuit


I am wanting to install a dedicated circuit for my amp, but getting from my panel to the wall behind my amp is going to be difficult. As I was investigating in my attic, I realized that when I changed my stove from electric to gas, the unused 30 amp Romex 10/3 wire will reach within 15 feet of where I want the wall outlet. Do I install a junction box and run Romex 10/2 to the new wall outlet and use the 30 amp breaker? What is the best way to connect the wires in the junction box?
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So I will change the 30 amp twin breaker to a 20 amp twin breaker. If I install two 20 amp Porter Port outlets and use 1 hot of the Romex 10/3 to each outlet (red and black) sharing the common white wire, will that defeat the purpose of having 2 dedicated 20 amp circuits. By the way, the stovetop was a cheap radiant electric cooktop and was a piece of crap.
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