I installed the Boom rear speakers myself without doing any type of 4 channel audio upgrade. I noticed a positive difference in the sound quality. About 1 month later I had the dealer install the Boom amp and front Boom speakers. The system sounds awesome, even at 80 mph. I was not charged for any type of 4 channel audio upgrade. I have seen the mention of the 4 channel audio upgrade on the website when looking at the Boom components, so I understand what you are talking about.
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2012 RoadGlide Ultra (Ms. Remedy)
D&D Fat Cat
SE Race Tuner
Performance Machine Super Gas AC
Boom Audio Amp
Boom Audio Speakers
HD LED Tour PAC lighting