#1
I have an AC15c1 and I've been offered a Marshall 4x12 extension cab for a really good price. My amp is 16ohms but the cab is 8ohms. As far as I understand (which is very little) this shouldn't be an issue for a tube amp. However the amp has a switch on the back to go from 16 to 8ohms.
The manual states:

16 OHMS: Combos allow for the following configurations:
1) Set it to this if you use the internal speakers only.
2) Set it to this if you connect an external 16 Ohm speaker cabinet through the
External jack.
8 OHMS: Combos allow for the following configurations:
1) Set it to this if you connect an extension cabinet through the Extension jack.
You’ll be running the internal and external speakers in parallel. The impedance of
the extension cabinet must be 16 Ohms.
2) Set it to this if you connect an external 8 ohm speaker cabinet through the
External jack.

Am I right in thinking that I would use the 8ohms switch with the extension cab?
Hopefully this makes sense!

Some reference:
https://www.ultimate-guitar.com/forum/showthread.php?t=1186899
http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/
Last edited by Nemesis117 at Feb 23, 2017,