I thought about doing the same - my 2009 H36 has the original stereo in it which luckily has an Aux, but no Bluetooth. At least with an Aux you can plug a battery powered Bluetooth device into the aux and send your phone/ipad music to the stereo that way.
But I decided the cockpit speakers aimed straight down at the helm - even after replacing - sounded awful. In the end I bought two portable, battery powered Bluetooth Bose Color Soundlink II speakers and they go up against the dodger, aimed back at the cockpit. They run all day, connected to my (or anyone else's) phone, and stream music beautifully. The two of them can be paired (otherwise known as Party Mode) and placed in stereo mode in the Bose app. I recharge them overnight, and they're ready to go again all day the next day. And, since they're portable, they sound great in the cabin at night with a movie, dinner music, etc. No rewiring necessary. And you can control the music from anywhere. Bonus: You can take phone calls thru the speakers as well - it will pause the music and connect your phone call automatically.
I eventually disconnected the awful cockpit speakers in order to make room in the arch wire run for solar panel wires, but left the old stereo connected to the speakers in the cabin. Reality is we NEVER use it, since we stream music to the portable Bose speakers when we want it. The stereo is connected to XM/Sirius, but since you can stream that on your phone, why use the old stereo?
YMMV. We love this modern solution and can't see going back. Just a thought for others who are contemplating upgrading the old stereo system and haven't started drilling yet!