You are buying these bookshelf speakers and a stereo amplifier to actually listen to music, not to replace them in two years when a wire frays or a tweeter blows. A good hi-fi setup should last a decade or more, so we want to buy the right gear the first time and avoid cheap gear that leaves you wanting an upgrade by next winter.
Late November is the sweet spot for hi-fi equipment sales because manufacturers drop prices right before the holidays to clear inventory. Look for last year's amplifier or speaker models that are identical in sound quality to the new releases but discounted just because a newer box sits next to them on the shelf.
The 0% intro APR is a deadline, not free money, and if any of the $550 balance is still sitting there when the 15 months are up, standard interest kicks in on the whole original amount.