As an aside, using an AC supply has several advantages to it.
First, as with any audio appliance that has the power transformer external to the chassis and audio circuitry, you get the hum-reduction advantages (i.e., the transformer is not close enough to the audio stuff to influence it).
Second, the hardware required to convert the AC to DC is minimal, in terms of footprint, cost and weight (although there is the issue of heat dissipation on the regulating semiconductors, I suppose, if current draw is high).
Third, the circuit is powered by known regulation, as opposed to some black epoxy-encapsulated thing which purports to provide DC but one has no idea how ripple-free it may be.
Fourth, the AC can be readily converted to several DC voltages, although I suppose DC can be downregulated too.
Fifth, there is no issue regarding the polarity of the adaptor plug/jack since both contacts are equivalent.