I ultimately purchased a Logitech Wireless Boombox. This thing is awesome. And loud. And portable. If I'm at the park playing sloshball with my friends, I can put this thing by home plate, yet it's loud enough that the people in the outfield are dancing and spilling their beer everywhere. If I'm at the beach and it's windy as fuck, I can still turn it to a volume (max isn't even necessary) such that I can hear the music just fine from a distance. Oh and you can link the speaker to your phone via bluetooth to play music wirelessly. There is also a 3.5mm cord too if you want to do it the old fashioned way. The manual and reviews keep bringing up the iPhone but you can pair it with any smartphone with bluetooth that has that AD2P profile.
Here it is on Amazon. 1300 reviews and counting, the vast majority of them 5/5 star reviews.
You can find it cheaper on ebay for about $100 brand new these days. There are no other portable speakers on there for $100 that perform like this thing. One important thing to note, is that when it is unplugged, the bass and max volume is reduced. It will still be more than loud enough for your 10X30 campsite. If you want to crank it up though and have max volume and bass yet be nowhere near a wall outlet, there is a solution.
Buy this extended battery. (and don't bother getting a solar charger for your phone)
At the time of typing this up, it's on Newegg and Amazon for $60
If you are getting this to use with the Logitech Wireless Boombox, (instead of just charging phones) You want the Anker Astro3, NOT the Astro3E!!!!! If you accidentally ordered the Astro3E because i put up the wrong newegg link yesterday, and want it to be able to power the Logitech, cancel that shit and reorder the correct one! If you're just using the battery to charge devices then you can probably keep the Astro3E
You can plug the Logitech Wireless Boombox into this Anker 10000 mAh battery and that thing provides enough juice that the speaker thinks it's plugged into a wall and operates at max volume and bass. Which is awesome. If you're going to buy the Logitech, you might as well get this Anker battery too because at the very least, it will double the life of how long the speaker lasts. If the Logitech is dead, the Anker will charge the Logitech while you play music at the same time, it's plenty powerful enough. There's also the added benefit of getting max volume and bass with the Anker, should you want to annoy anyone within 10 campsites of whatever direction the speaker is facing. The best benefit of all, however, is the fact that you can charge multiple devices at once with it. You can blast your tunes at max volume AND charge your smartphone. Or 2 smartphones at the same time. Whatever you want; it has 2 USB ports on it. I would recommend this Anker battery to any campers even if you don't buy the speakers because you can charge your phone and your friends phones at the same time, multiple multiple times over before it runs out of juice, which is a whole hell of a lot more convenient than using a solar charger that costs about the same or more (I see a lot of solar chargers in the $60 range). Just get this battery, it will definitely charge your phone all weekend and means you don't have to run your car or sit at a charger station with a bunch of other people for 2 hours. If you are using the battery for the boombox, however, you'll more than likely have to charge the battery once or twice to make it the weekend if you're constantly using it, but a minor inconvenience for what you get out of it.
All in all, get the Logitech Wireless Boombox. And if you get the Boombox get the Anker 10000 mAh battery. Even if you don't need the max volume and bass at your campsite you'll like it just for extended battery life and the ability to charge your phone too. And once you leave Coachella and use your Boombox somewhere else like the beach you'll be glad you have the battery to make it louder.