well it sounds like this amp has 2 seperate poweramps each capable of producing 575 watts each at 4 ohms.
its my guess there's a switch to engage a crossover to send highs to one amp and lows to another, or when it's off each amp gets the full range signal (my preference). most people mean the former whan they say biamping, but technically when you use two amps full range you're bi-amping as well.
anyhow, check out the ohms faq linked to the faq sticky at the top of the forum for adetailed exlination of this but here goes. yes, in order to get the full 575 watts out of each side you need to plug in a 4 ohm load to it, which will be either one 4 ohm cab or two 8 ohm cabs. an 8 ohm cab by itself will probably still get 300 or so watts, which will still be pretty darned loud. other factors like the number and size of the drivers in the cab, the sensitivity of the cab and the overall voicing of the cab will affect how "loud" it sounds regardless of impedance