I suppose I could, but I’d still rather repair such an incredibly rare device if possible.
I’m probably flat out wrong but, could it be that the input is improperly configured?
Alternatively, does the speaker also sound quiet if you directly (as in, bypassing the transformer) connect it to an amplifier?
Either it’s the transformer dampening the signal, or there’s something wrong with the voice coil itself not being able to move as freely as it’s supposed to, which could explain why it’s sounding quieter than it is supposed to.
What do you mean by “improper input”?
I believe I once tried connecting the speaker directly to an audio source & I think it was still quiet (which means that there’s a problem with the speaker & not the circuitry it’s attached to).
I meant that the transformer could be somehow dampening the signal as it normally expects a higher voltage than your amplifier is sending (sorry, it was 2 a.m when I wrote that)
But that doesn’t seem to be the case since you mention that even by bypassing the transformer it sounds quiet, I can’t think of anything else besides the voice coil having gotten jammed.
I’m afraid I got nothing besides that 
I’ve done the same thing with all my other speakers, some of which have transformers, & that hasn’t happened (that is, they’re not super-quiet).
Yeah. Guess the only thing that could be done would be to replace the speaker right? (even though I can’t do that for the above reasons)
Unless there was a way to take off the original speaker (it does look like that it’s glued on the faceplate, I couldn’t see any clips or anything like that) and fitting a replacement speaker, I’m afraid this one might be a lost cause.