(I’ve also included a few others brands you might encounter as aftermarket installations.) This should help you identify your speaker.You also mentioned that you’re looking for a speaker option that decreases the overall volume of your amp.Their G12C/S speaker is listed as 99.79 d B, while their G12C is listed at 96.10 d B.The G12C is more than three d B lower, which equates to approximately half the loudness of the C/S.
The SPL rating specifies how loud a speaker is at a distance of one meter when driven by one watt. I don’t know what speaker is in your amp, and even if I did, its specs might not be available.
That’s probably a moot point, though—experience tells me that most original speakers in amps of that age are substantially fatigued and generally sound very weak. It's a great-sounding speaker, but with a sensitivity rating of 100 d B, it’s relatively efficient and would probably be pretty loud in that amp.
Let’s take a look at a couple speakers from Warehouse Guitar Speakers (wgs4.com) so I can better explain this.
Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged.
If you are unsure of this procedure, consult your local amp tech.