Leslie 16 / Fender Vibratone help needed please..

So recently I bought a '67-'69 Leslie 16. (similar to a Fender Vibratone.)

The thing works great but it doesn't have an original speaker.
It has a big 15 inch speaker in it which says JBL 100 watt on the back of the speakermagnet. I do NOT know the impedance of the speaker.

I want to use this with a old '73 100 watt Marshall Superlead and a old 4x12 which is loaded with 4 G12H30 Greenbacks.
(Yeah, I know, not quite the amp to talk about on a Fenderforum)
Now, I don't want to ruin the outputtransformer in the Marshall nor damage the Leslie.

Question:
Is it wrong to use the 100 watt Marshallamp with this 15 inch lesliespeaker while not knowing the impedance of the 15 inch lesliespeaker? Should I change the impedance on the amp? If I would change out the speaker in the Leslie for a 12inch: What would the ideal speaker be with this setup? (As in: Watts, Ohms, Hrz)

I hope you guys can help me out!
Thanx in advance!!

Oh boy......where to begin?

I had one of these thangs "back in the day".

If Agent Orange hasn't polished off the last of my gray matter, I recollect that the stock speaker in these units was a Fender "special design" 10-inch (prolly a Utah but could also be an Eminence or CTS I s'pose).

This cab was intended to be plugged into the main speaker jack of the parent amp, and the amp's speakers were then connected to an output jack on the rear panel of the Leslie itseff. An internal bandpass filter channeled the lows and highs to the amp's reg'lar speakers, with the scooped mids fed to the Leslie's driver (a bypass footswitch was provided to switch the unit out of the output signal path for normal audio). I am thinkin' that the Leslie's ten was 4Ω but I never disemboweled mine to actually look.

From what I gather from your post, it sounds like a previous owner made some serious upgrades to the basic unit. The first thang I would do is measure the impedance of that 15-inch driver with a DVM to confirm what you actually got. Then mebbe trine find some additional info on the web or wherever. Consulting with your local amp tech might not be a bad idea either.

I'm curious about the speaker currently in the cab -- every professional-series JBL I've seen has the impedance marked on the foil-cal attached to the magnet cover. Mebbe you could post a pic or two......?

Best of luck, those Vibratones rawk.

:band