I purchased the stebel magnum horns to replace my worn out tiny beepers on the project bike. (As far as I can tell, they are only available for order in the US here:
http://www.bikerhiway.com/stebel-magnum-motorcycle-horns-dual-tone-chrome-p-280.html) This was my first foray into anything electrical that didn't HAVE to be done, so it was a big step for me. Let me tell you, these horns are the BALLS.
They rate 3 db lower than the Stebel Nautilus. My understanding: Since db are logarithmic, 3 db means it's half as loud. But it's still heads and tails better than stock. I haven't compared side by side to my truck, but these things blow, baby!
I picked these mainly because the bike is a cruiser, and being naked there really wasn't a good place to mount the nautilus that I could see. With these however, they were smaller and easier to work with, I mounted them on the front of the frame below the triple tree. According to the cardboard mock up of the nautilus I made, I may have had a "conflict" between the horn and my brake lines under braking, which I definitely didnt want. The magnums consist of one high tone and one low tone, cheaply chromed, trumpet horns, running off voltage only (no air). I'm not sure they make the most appealing fashion statement, but they definitely work where they are. I tried multiple other places but they were either two close to the headers or two close to the valve cover for comfort, since I didn't want melting horns.
Installation kind of sucked because the directions were less than clear, and I'm an electrical noob. It was my first time using a relay: a wire runs straight from the positive battery terminal, through an inline fuse, into the relay. The old horn wires run into the relay as well. When the horn button is pressed, the old horn buttons trigger the relay, opening up the switch and allowing power to the horns, which are grounded to the chassis.
Anyway, I didn't want to **** off the neighbors by honking anymore, so I'm done for now. But I can't wait to try it out some more another day, with a comparison to the stock sv650 horn and my truck's horn.