I want to run my Line 6 Spider Valve combo through an 8 ohm cab, and will run it through the 8 ohm output on the back(there are 3, one to the main speaker in the combo, one for a 4 ohm, and one for an 8 ohm)

I want to disconnect my speaker while I run it through the cab, but I dont want to **** anyting up, is it safe? I know youre not supposed to run the amp without a load.
If you plug a speaker lead into the output it (should) bypass the speaker in the combo.

Check the instructions it should say if that is the case. Most modern amps have the combo speaker bypassed if external output is connected.

If you want more power you should be running out the 4 ohm output into a 4 ohm cab.