So I was thinking of buying the Acoustic B200H head and B115 cabinet. I'm quite new to the world of seperate heads and cabs so I was wondering if the head has a switch that changes it's output from 4 ohms to 8 ohms because I remember reading that the total speaker ohms should equal the head's ohms.

Also, can anyone describe the tone of acoustic's amps or if they have the exact set up, the tone it has?(I'll be playing with metal guitarists mostly so it'll need to cut through somewhat, though I'd definetly want versatility)
The head won't have a switch. It will put out a certain amount of wattage going into 8 ohms (probably ~130 watts in the case of your amp) and full wattage going into 4 ohms (200 watts for you). Some heads have different minimum ohmages, but for the most part it's 4 ohms. Tube amps will sometimes have a switch and put out the same wattage no matter what the ohmage because they require specific ohm settings to work properly. But, your amp is not a tube amp so will not have a switch.