If I'm reading you right, you want to drive the cab with the Spider...
If the Spider doesn't have an external speaker out you will need to break into the speaker wire, install a male/female set of jacks (male end to the powered side, female to the speaker side, and also a female on the cab if not already there) The tip of the male jack will be "+" , don't use a guitar cable for the speaker extension and also match the load impeadance. This description allows for one or the other, not both.
Hope this helps.
I did the same with my Traynor combo to a 2x12 cab. worked fine.