bassjoc

hi there ive just started playing bass and some issues regarding amps have left me very confused! i have an ashdown mag evo II 300 watt 2x10. The speakers in the combo are 80 watt @ 16ohms (making it 160 watt @ 8ohms right? or wrong?) The 1x15" ext. is 250 watts @ 8 Ohms so when its connected i effectively have a 300 watt head running a 410 watts cab (@ 4ohms) . Again im not too sure how this works so any sort of guidance would be greatly appreciated. How many watts am i realistically getting out of the combo without the extension, is 250 watts the peak or rms of the extension and would a 1x12 250 watt 8ohm extension do the same job???? Cheers guys