I've owned a number of different outdoor speakers and listened to quite a few others most of which have already been mentioned. I can say without a doubt that for my ears, the Aperion audio Intimus outdoor speakers are the best sounding I've found so far. Smooth highs, good mids, really decent bass for an outdoor listening environment. They are $149 each and best part is if you don't like 'em, ship them back for a refund. It doesn't get much better than that.
Aperion Intimus Speakers
These things have great bass providing you mount them up under the soffit next to the exterior wall of your house.
If you look at the pic, these have a -3db 0 +3db switch on the back of the speaker. That helps you tune the bass to your listening environment.

What I think they did (I haven't opened up the cases) is use a voice coil woofer. On half of the voice coil they run it traditionally. On the other half they have it cut in around 80Hz or so making it a quasi-2 1/2 way speaker so you get better bass outdoors. Obviously they have a padding circuit for the level adjustment.
Pretty clever design if in fact that is what they do.
I use mine on our back deck with a Squeezebox and a little Jolida 1301A 30wpc hybrid. The deck system can kick it pretty well considering. It even images.
Well, there's my $.02
