Thanks for the replies and advice.

The combo amp is factory configured so that the internal speaker is disconnected
if using any of the rear extension speaker outputs.
So that's why I need a 'workaroud' alternative option.

Now I've had a chance to look closer inside the combo,
I'm happily surprised to find the on-board 16 Ohms speaker is not hardwired to the circuit
as I presumed,
but connected by it's own dedicated internal jack socket
hidden away behind the valves in a dark corner out of sight under the chassis.
So I reckon I can get away without need to buy or make up a "Y" splitter lead.
The external 16 Ohms cab has input and output jacks;
so I should be able to run a lead from the combo's rear 8 Ohms output to the extension cab,
then disconnect the combo's internal 16 Ohms speaker, and plug it into the passive output of the speaker cab
using a male to female jack extension speaker lead.

Instant mini stack !!!!

1x10" open back 5 watt valve combo sat on top of matching 1x12" 50 watt sealed back cab.


Should be interesting;

[reasonable quality OEM speakers, so no available tech specs for 'efficiency']
.. I'm guessing the smaller brighter toned 10" speaker [unknown wattage, but at least sufficent for purpose]
will begin to crunch nicely
while the 50 watt 12" cab remains clean and beefy..

.. and I can connect up a speaker emulated power attenuator/line-out -> DI box to the frontline mixer whenever necessary...