Bear with me and I'll try not to go on for too long.
I spent yesterday afternoon replacing the OEM 5.25" RG speakers in my 2015. The actual speaker replacement was a breeze but I spent hours messing with three different solutions to sound dampening and insulating the inner fairing where the speakers mount.
I'm using the JBL GT0638 speakers in my fairing and HD Headunit (4.3) no amp. All of my observations are subjective, with the bike sitting in the shop. Listened without the bike running and then with the bike running at idle. No road tests yet (weather is holding me back)
EQ settings were flat lined and speed volume set to off.
First solution: JBL speakers installed - no mods other than speaker swap. The new speakers sound a bit better than the factory 5.25" speakers. Still sounded tinny, mids were nice and bass was a little improved but still boomy.
Second solution: I used Dynamat to create a backing and barrier between the back of the speaker opening and all of the other open space in the fairing behind the speaker mount. I'm no audiophile but the treble was tamed down and sounded more natural. Mids were still very good and the bass response sounded tighter.
Third solution: Dynamat removed and I created a new barrier between the back of the speaker opening and all of the other open space in the fairing behind the speaker mount using foil backed HVAC tape (.127 inch thickness). This caused shrill high frequencies, didn't affect the mid range much, and I think it actually made the bass worse.
Fourth solution: HVAC tape removed (note to myself - this stuff is designed to stick when used on stainless or galvanized HVAC ducting - but it could be used to hold plastic pieces together forever) Once this stuff sticks to plastic it is a chore to remove
I tore open an old polyfill pillow and removed the stuffing. I started to fill the enclosure with the polyfill, but began to worry about all of the fine fibers that were going to blow around inside the fairing long term. So I pulled the first batch of stuffing. At this point I was about ready to quit and just put everything back together and call the new speakers good without doing anything inside the fairing or speaker enclosure. But persistence pervailed.....
I went into the house and grabbed two large ziplock freezer bags. I stuffed the freezer bags with polyfill and then placed them inside the fairing and speaker mount. This pretty much shut off all of the other open space between the inner fairing and speaker mount. Then I reinstalled the JBL speakers and buttoned everything back up.
It might be a placebo effect since I spent so much time on this, but to me I think this solution sounds the best with these particular speakers. Treble was more natural without ringing your ears. The tinny sound is gone. Mid range still sounds great and the bass response sounds tighter. It doesn't sound like the drums are coming out of a cave, not boomy at low or medium volume levels (haven't pushed the volume yet since by this point the wife was already in bed and probably wouldn't appreciate Rob Zombie blaring out of the shop).
At this point I"m calling the whole thing done. I was going to add an AMP next weekend, but now I'm going to wait until my patience comes back. During the last speaker install I lost two mounting screws down in the fairing. I managed to fish one out from the bottom of the fairing but the other one is lost forever to the motorcycle gods. One last observation - those speaker screws can't be picked up or fished out with a magnet pen. I'm not sure whether it's the coating or the substrate.