You absolutely DO have to have the amp for a CVO Road Glide Ultra. The directions are so vague my local dealer's tech had to call and open a case with Harley because when he went to flash it, he could not add the 6 or 8 speaker option. The RGU has a Stage 2 Boom system, and anything with a Stage 2 system has to have an amp.
Your saying you have stage 2 and another amp is is needed for the fairing speakers?? Or ?? A lill confused ....
There's 3 stages of the Boom system.
Standard - No amp
Stage 1 - One 300 watt amp
Stage 2 - Two 300 watt amps with the stage 2 speakers.
RGU and the Limited both have the Stage I system with only one 300 watt amp and stage I speakers. On either of these bikes you would need to add another amp to power the lower speakers. Kiss one of your saddlebags goodbye as it's going to take up a little room there.
Other option is to find a 6 channel amp that will fit under the radio replacing the stock amp that's in there now but you will not have fader controls.
Even a Stage 2 bike like the SESG that already has 2 amps still needs a 3rd amp added for those lower speakers.