I do this all the time. What you will need is a cable that is 1/8 inch on both ends, and a 1/8 inch to 1/4 inch adapter.
Plug one end of the 1/8 inch cable into the headphone out on your usb device. Put the adapter on the other end, Plug it into the effect loop return.
Plug your bass in as normal. Now you can play to the .mp3.
If you dont have an effects loop. You can use a line in, .mp3 in on your amp.
If your amp has two instrument inputs you can use one of those.
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