When you say 2 x 8 ohm outputs, do you mean two speaker outputs each with a minimum 4 ohm load but with a minimum 4 ohm load overall, or do you mean two outputs each of which have a minimum 8 ohm load? If the former, you'll get the full whack from one output driving a 4 ohm cab, unless it's an Ashdown Superfly with two separate 250W power amp modules. If the latter, you'll simply blow it up if you connect the cab to the amp, regardless of how many cables you use.