If it has a ine leve in, use a line out from the board to that. If not, if the combo has an effects loop, use the return. Either way, keep the line out volume down on the board and keep the amp off with both volumes input gain and master down until you're all hooked up.
Turn the amp on and bring the board up a little, then bring the master vol up on the combo if you don't hear anything.
Duoble check the wiring a couple of times to be sure that you have the correct line out on the board and it is sending signal.
Go sloowww! What you don't want to do is send the power amp in your combo some nasty big transient from plugging/unplugging or excesive gain by missing a setting on the board.
I have heard that a passive DI will convert in reverse so something cheap like a GT ditto or passive whirlwind, etc might work if the line out is coming down a snake. In that case you would go line out to snake, snake to DI mic output then 1/4" to the instrument input on the combo. You would also want to follow the advice above about double checking and going slow...
Hmm, I might hve to go downstairs and try that out...
