| Hey Krispy, are you sure the farting sound is coming from your cab? What did you eat for lunch?
OK, you have an Ashdown Mag 300 which gives 307 watts at 4 ohms. You also have 2 cabinets: a 2x10 and a 1x15. These cabs are rated at 8 ohms each, right? Plug each cabinet directly into the Mag head, and your amp will then be giving you full power at 4 ohms. This is parallel.
If you plug only one of your two cabs in, your amp will run with an 8 ohm load, and it will only be giving about 200 watts of power.
If you somehow have your cabinets hooked up in series, your amp will be running at 16 ohms, and you will have about 130 watts. This will sound like crap.
Parallel lets more power flow to the speakers (less resistance) and series does not allow as much power to the speakers (more resistance).
Think of parallel and series like this: Your amp is a rain-barrel full of water, and your cabinets are 2 - 20 foot garden hoses.
If you connect just one hose to the bottom of the barrel, water will run out at a nice easy pace, 20 feet away from the barrel.
If you hook both hoses directly into the bottom of the barrel, water will come gushing out, 20 feet away from the barrel.
If you connect both hoses together, and one end of your double-length hose to the barrel, water will come trickling out 40 feet away from the barell. | 
