You want a speaker that survives the elements and keeps the bass heavy for every summer hangout. Buy quality now so you aren't replacing a cheap, broken unit in six months.
Check the release dates for the brand you want. Companies usually drop new models in the spring, which means last year’s version often gets a steep discount for the exact same performance.
Remember that the 0% intro APR is a limited window. If you haven't paid off the full $300 balance by the end of the 15 months, you will start owing interest on whatever is left.