I have dual FIAMM Freeway Blasters (high and low tone). To make sure they went together well and were maximally loud, I didn't just add the low tone to the stock horn, I replaced the stock horn with their high tone one. Good sound, vast improvement over stock.
I don't think I'm going to bother upgrading because I doubt anything more than that would work well without wiring a relay and power from the battery which I'm just too lazy to do, but quad tone would be the way to go. The easiest way would be to buy the old quad GM horn set, for example
http://www.ebay.com/itm/GM-CADILLAC-ELDO-DEVILLE-FOUR-4-HORNS-SET-TRAIN-4-NOTE-HORN-SET-A-C-D-F-LOUD/303141299929 ...
@DubC this is your jam.
Another way to get quad tone horns is to go PIAA. Like in my sig...
High-tone set
Amazon product ASIN B0060ZB43GLow-tone set
Amazon product ASIN B00079V5ZIalso, sound clips of them in my sig, though unfortunately not together. There was a YouTube vid but I can't find it anymore.
That Marco one is interesting, though, I may have to check that out.
I saw a post where a Miata posted one of these cross-car low in front of the rad:
Amazon product ASIN B01GEQQ0BQand if you have enough space you could do these:
Amazon product ASIN B018WIM626Probably not as great as real pneumatic horns but a lot easier to install.