MM 210 HD 150 and cab

ok so...
I want to use a 2x12 cab simultaneously with the 2x10 speakers already built in my MM 210HD 150. What should the impedence be on the cab in order to utilize the amp's full power?

Well, look on the back of your amp at the extension jack. Somewhere there it will say "these outlets are wired in series" or "these outlets are wired in parallel". If they are wired in series, put it on 8 ohms, if they are wired in parallel, put it on 4 ohms. Just can't remember what the 150's do- - - - or lll.