One solution is to get something with substantially more oomph... like an Ampeg 1,000 watter going through a 4x10 or 6x10 cab. :D *heh heh heh SNORT* Something like a 4x10 with a big horn in the middle might also do the trick... though those'r 'spensive.
Also, the fact that you're running through a '15. '15s are great for straight-ahead fingerstyle but are crap for slap and for pick... or for if you chord rake like Dave Murray (Iron Maiden). The '15 might not have the requisite punch for this outdoor setting you describe. A 4x10 might do the trick instead. An excellent compromise is a 2x12, but those are hard to find and generally ain't cheap.
Might also have to put it on unusual settings... i.e. more mid and treble than you'd normally like.
So... more watts, more and smaller speakers!!!