I'm using an original, with original Fender tubes, Blues Deville 410 with my current band. During practice the amp does just fine but live it needs a little bit more without running the volume up to the 8/9 area.

I understand it has an external speaker output for running an extension I believe, and is rated at 4 ohm (I could be wrong, haven't checked specs in a while). So my thoughts are using a 2x12 extension cabinet. A friend of mine has an unloaded pine cab that I'd just need to load some speakers in.

My question is that since I'm a complete novice to amp impedance, before I go destorying everything what impedence should I be looking for when loading 2 12" speakers in the cab to use with my 410? My understanding is to equal 8 ohm for the 410 to go into, but other opinions are valuable. So in other words; 2 12" speakers rated at 4 ohm each?

All help is appreciative and I apologize if this is a silly question but I just want to make sure everything will work correctly with my setup. And and specific speaker suggestions for this is also a plus. Thank you!
First find out exactly what is needed load wise or you can destroy the OT. Is there an ohm's switch?
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No load switch that I'm aware of. From what I'm reading the output is 4 ohm meaning it has 2 8 ohm speakers so I'd need a cabinet with 2 16 ohm speakers for an output of 8 ohm. Equalling a 4 ohm amp through an 8 ohm cabinet...does that sound correct?

I've never used a combo through a cabinet before, just standard guitar head through cabinet.
The Deville is WAY loud on it's own and I'm not sure what the 4x10 ohm rating is so measure it first. The amp is good down to 2 ohm so not much chance of blowing it up. Personally I would point the amp at my head instead of my knees and mic it for large venues. Better coverage with less gear to drag around.

My rule of thumb: If they mic the drums, mic the amp.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY