OT Marshall amp + external spk

["Michael Peters" <mp@mpeters.de>]

here's a technical question that is not about looping:

In my band's rehearsal room, I use a Marshall combo (JCM2000 DSL401) which
sits on top of an ancient Fender Vibratone leslie
(http://www.nmia.com/~vrbass/vibratone). The leslie was something I used a
lot in the 70s (as 2nd speaker) but it has gathered dust since. I'd love to
use it again now that we have a larger room and it there is enough space to
whirl the sound around.

I tried plugging it into the Marshall once - not only was the resulting
volume lower but it also sounded somehow _not right_.  I didn't dare using
it like that because I was afraid to damage the amp. Can someone tell me 
how
to set it up?  Is it possible at all to use the leslie with the Marshall?

The leslie (according to the vibratone page) has a 4 ohm speaker. 

The combo amp has two outputs one of which is plugged into its 16 ohm combo
internal speaker. The other output is unused. The outputs are labelled with
"40 W RMS into 8/16 ohm".  There is a switch that says "8 / 16 ohm" and it
is set to 16 ohm.

-Michael