There's at least one, maybe two things wrong with your setup:

• "The amp is connected to the cabinet through a short guitar cable."
  

You can't possibly get the high-current drive that speakers need through a guitar cable. A piece of #14 lamp cord (also called twinlead) is much better suited to the task. A guitar cable is meant to carry a few millivolts at a few microamps of current. At 120 Watts, an 8 Ohm cab needs something close to 30 Volts at about 4 Amps. A guitar cable can't do it for you.
• I'm not familiar with those Peavey cabs, but some old Ampegs used a special cable and connectors to provide an interlock protection for the amp. If the output was not connected to a suitable cab via the proper cable and connector, the amp would not produce enough power to hurt itself. You may find that you need a Peavey speaker cable to connect your amp to your cab. The connector you describe sounds like something called a Speakon connector. A generic Speakon cable may do the trick, but it wouldn't hurt to get a Peavey part.

Either way, toss that guitar cord!