Most efficient/quietest way to drop DC
Since it is an old hifi, I suggest to use a linear regulator and not a switching device / buck converter to avoid noise pickup into the old hifi circuits. You need to be able to loose 3W of energy in the regulator on the other hand. Find a high input voltage regulator in a TO220 enclosure and apply a proper heatsink. To avoid noise from the bluetooth circuit add a LC filter (eg. 330uH and 1000uF) in front of the regulator and keep the antenna as far away as possible from other circuits.