Meh, the claim that SMPSes produce too much noise to be used in audio is overblown, IMO. Sure ripple and noise from a hard-switched SMPS are going to be worse than the typical linear power supply, but given that the switching frequency will invariably be well above 20kHz it won't be audible even if PSRR of the amplifier has dropped to 0 by then.
The real problem with SMPSes in this application is the potential for interaction between two control loops - the voltage regulation loop in the switcher and the negative feedback loop in the amplifier. Pretty much every car audio amplifier sidesteps this problem by running the switcher open-loop, and that's not a bad solution, really; I would suggest always using pulse-by-pulse current limiting, however.