Thanks for your help Jim.
The info about the tubes is interesting, I might try a pair of those 6550s and the 12AT7 for the pre...
Yeah, the output on the clavia is certainly not adequate to drive the Leslie amp, but since I wrote last, I have used a couple of different preamps in front of the Leslie amp and that seemed to give it much more punch. I tried a Peavey T-Raxx bass preamp which I found in the garage the other day... but I think it needs a service as it was making some weird rumbling noises every so often... then I tried a little Joe Meek VC3 (which is a vocal mic preamp?) which gave me much more gain, and some compression before going into the cab. Those speakeasy pedals are a bit expensive, and it's tricky trying to find someone who has one I can borrow in the UK... but I will perservere finding one to try.
I will look into the replacement horn though, as the cab does have a sort of 'buzzy' distorted sound around the midrange section. I tried listening to the top horn independantly to the bottom one by disconnecting the speaker wire, and it does sound like it may be blown.
anyway, I have a few more photos for you which may shed some light on the wiring route and the mystery box!