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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Your stove should be on a 40 amp circuit, which requires 8/3 Romex cable. Change the breaker from the 40 amp twin breaker to a 20 amp twin breaker then make the stove receptacle box a junction box where you splice some 12/3 Romex to the 8/3 Romex and then run the 12/3 to a duplex 20 amp receptacle with the tie on the hot side removed. This will give you two 20 amp receptacles. 
It is definitely a 30 amp twin breaker with 10/3 (orange) Romex. Do I need to change it to a 20 amp breaker? Is it ok to use the 10 gauge wire with the 20 amp breaker to a 20 amp Porter Port outlet?
You can use 10/3 with a 20 amp you can always go larger gauge but never go smaller. 
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