No you cant just plug in the output of an "amplifier" to the input of another amplifier.
It might work, but there would be an amazing amount of distortion not to mention a good chance of blowing something up. Think a mesa boogie with the lead gain and masters at 10! Then multiply that by 100 and you might be in the range!
What you want to do is to disconnect the speaker from the head of the combo and connect it to the head of the new amp under test. Chances are the speaker will unplug from the combo amp head, and you may need an adapter cable to connect the new head to the combo amp speaker.
You already have me worried and I am going to suggest that you might want to get some competent help to help you.