Question about an extension 2x12 cabinet for my JCM900 please!

[vklobucar]

I have a line on a 2x12 Marshall cabinet and I want to use it with my stock JCM900.

Enclosed is a picture of my amps speaker input and ohms switch. Question is, can I use an extension cabinet TOGETHER WITH the stock speakers and what would that new cabinet's ohms need to be.

As well what is a fair price for a Marshall 2x12. I think its loaded with those stock GM speakers if I recall?

[Twitch]

You should set the speaker cab to the 8 ohm mono setting and set the amp to 4 ohm. If the cab is 16 ohm mono, then set it to 8 ohm stereo set the amp to 4 ohm and plug both amp outputs into each input on the cab(youll of course be unplugging the built in speaker of your amp in the second scenario)

[NatG]

I'm in the UK and I'd pay about 2/3 of the original selling price of the 2x12. Here they go for £300 new, so I'd expect to buy used for around £200 for one in near mint condition. If it were damaged then maybe 1/2 the price. Whatever price they are in Canada just take 1/3 to 1/2 off the price they go for in shops.