Just a curious difference I spotted between my 9191E and 9191XE. The back of the woofer is physically different between the two models and they have slightly different part numbers.
The 9191XE has woofers with 16PB234P on them and the back of the speaker is "flat" just like every picture of them I've seen online.
The 9191E has woofers with 16PB244P on them and the back has an extra bit on the back which I can only describe as a large square bracket. Here is a video showing the back.
In terms of age the 9191XE may be earlier. It has a tape motor sticker that states July 1978. The 9191E has December 1978.
The 9191 Service Manual show the bracket type speaker in the exploded diagram and the part number is VSP0016P-244P matching the possibly later 9191E.
Maybe Sharp had a bunch of previous model speakers they used in earlier 9191's?
What got me curious was the obvious difference in bass between the two models. The later model speakers are deeper, maybe I will swap the front cases around to test the theory and see if the deeper bass moves to the 9191XE.
UPDATE: Found this image: