I searched for this but sort of answered my question but I also didn't feel like it did...sorry to sound like an idiot but I'm just making sure I'm not using someone else's remedy to fix mine when we're using different equipment.

So my predicament: Jump to bottom paragraph to skip story*

I have been inactive in bands for a while because of work and life not allowing me time but my old band was asked to play a couple shows soon and we all thought why not. Unfortunately I sold most of my "show" gear and have a modest set up of some pedals and my Vox ac15cc1.

Now these specific shows I noticed are at American Legion/Moose lodge type places and they DO NOT MIC the amps. The bands these all have Orange stacks or Mesa stacks that stand their own; not 15 watt Vox amps. But I don't have money to buy a new amp so that isn't my answer.

My question is I have an old 1x12 speaker cab without a speaker, so would I be able to SAFELY run a 8 or 16 ohm speaker in this and run it from the extension jack on my Vox? There's an input and setting for 8 and 16 ohm but my impedance knowledge is rusty so I'm not quite sure what the best way to add a cab to my amp would be and at what ohm rating. Sorry for the redundant question it's just been a while since I had to match ohms and the threads I've seen aren't pertaining to my specific amp so I'm just playing it safe.
Depends on what's in your Vox.

If it's the original 16 ohm Wharfdale 12", then you could run a second 16 ohm speaker cabinet by plugging it into the 8 ohm output on the back of your cabinet (you're now running two 16 ohm speakers in parallel, which gives you a total of 8 ohm impedance).

There's an impedance switch on the Model AC15CC1
16 Ohm:
Allows for the following configurations:
1) Set it to this if you use the internal 16ohm speaker only or 16ohm external cabinet only.

8 Ohm:
Allows for the following configurations:
1) Set it to this if you connect an extension cabinet. You’ll be running the internal and external speaker in parallel. The impedance of the extension
cabinet must be 16 Ohms.
2) Set it to this if you connect an external 8 ohm cabinet and disconnect the internal cabinet.

You don't want to run two 8 ohm speakers, for example, or an 8 ohm speaker and a 16 ohm speaker together.
Worth noting that the second speaker isn't going to improve anything all that much (you won't be significantly louder, for example).
I think you'll find that 15 watts is marginal for this kind of gig, so your drummer (and the other band members) may have to adjust. You'll likely also find that your bottom end is a bit wimpy at show volumes. If you're going to purchase a speaker, get one that has high sensitivity (as in NOT a Celestion Rocket 50, which will make your 15 watt amp sound as if it has 3.75W).