Your loudspeakers are of vital importance to the proper functioning your amplifier. Tube amplifiers must be connected to a speaker load at all times. The briefest interruption while playing at loud volumes is enough to cause serious damage to your amp, maybe even enough to blow your output transformer. Very often the output tube sockets will arc over and need to be replaced.
Always test your speaker connection at low volume before you start playing loud. Roll the speaker cable on the floor with your foot and see if the sound level changes. If so immediately replace the cable. Replace loose or dirty speaker jacks with new Switchcraft types. If the speaker wires are exposed, as with Fender combos, check the edges of the crimp connections to see if the wires have become frayed or are broken.
Two parallel connections are better than one, so if you're able to hook up two separate cabs on a Marshall or Hiwatt head you will be that much safer. But on later Fender silverface combos, however, the external speaker is in series with the combo's speakers so there is no advantage. Modern cabs that have stereo-mono switching can be made more secure by removing the switching circuit. Hardwire your speakers to the speaker cabinet jack instead. Often, your sound will improve as well.
With all Marshall heads be sure to check the impedance connector. JMP's have a pullout jumper that should be cleaned with fine sandpaper or steel wool to remove corrosion. If the connector is loose, it may be best to completely bypass the impedance selector and have a tech hard-wire the output transformer directly to the amp's speaker-out jacks. With JCM800 heads the rotary speaker selector can often be set between two positions giving you no sound whatsoever. Take a screwdriver and turn the switch a few times to be sure that contact isn't dirty.
And lastly, it's a good idea to buy a good quality speaker cable, 18 gauge is really a minimum. I use 10 gauge, oxygen-free copper, multistranded speaker wire. (The smaller the number, the bigger the wire) You do hear a difference.