Sam - Before replacing or repairing, I'd do some more trouble shooting. an easy solution here would be to bypass the 420 and the source deck, by taking an iPod cable and plug directly into the amplifier and play directly into it. MAKE SURE YOUR IPOD VOLUME IS TURNED DOWN A BIT. This in effect, eliminates all potential problem area's up stream (meaning deck and 420). If you still have a problem, then yes, its the amplifier.
A word of caution about repairing amplifiers that have blown outputs or blown power supplies. Make sure you talk to the repair tech and find out if you have another problem with the same amp its covered and for how long. Especially if its a flat fee. Sometimes in current production models a mfg will just pull the board and put a new one in. Others repair the board. I suspect in this situation your looking at a board repair.