Lots of folks swear by the Denali Sound Bomb and associated bracketry. Costs about $70 from Revzilla and other vendors.
I watched a YouTube video that showed a guy adding this VEMO horn in place of his stock horn on his '14 GSA.
Being frugal wherever possible, I ordered one and gave it a try. The horn mounted in the stock place, the connector in the box connected to the stock harness connector and all was rosy. It was deeper and louder than stock but not earth shattering like the Denali is.
After a few hundred miles the horn became intermittent. I removed it and discovered that it had some water inside, likely due to my error in mounting with the bell facing forward and up. Duhhhh.
Having added the EZCan to my GSA I decided to use one of the High Power Circuits to power this horn and to reinstall the stock horn as well.
WOW! I mounted the Vemo Horn to the fairing mount in the same place the Denali Sound Bomb Horn gets mounted using the bracket that is supplied with the horn.
Once connected to the EZCan the optest was more than satisfactory. The first time I blew it in the garage my wife came running out to see what the matter was. She had been in the basement 50 feet and many rooms away.
This VEMO horn is much louder through the EZCan than it was when connected to the stock horn connection. The two horns together are quite satisfactory.
Horn - Low Tone - VEMO
BRAND: VEMO PART#: V20-77-0005 $10.95
VEMO BMW Horn