I've run Magnums in all of my bikes .
I found the stock wiring on the Deauville was so weedy that it dropped a fair bit of voltage, and hence the horn was somewhat weedy and quite. But (re)wiring it through a relay and properly spec'd wire (25A cable and not bellwire) brought it up a fair bit in level. Another one (so you get a high and a low tone) makes it easier for people to figure out where the noise is coming from - but you'll definately need a relay.
I'm now on a Nautilus - it's very much louder than the stock horn, I think - but again it needs a relay.
'HOWTO' guides on wiring a relay for your horn are plentiful on the internet...