I'm sure to a lot of you this is going to sound stupid. I recently bought a Marshall Valvestate 150w combo amp and I know it's possible to get an extension cab to make it louder. Does it have to be a specific cab or can I plug it into any 4x12? If it's possible to put it through any cab, how?
what does it say next to the speaker jack?

But typically your safe with a SS amp, but you deff should check.

You can look on most amp sites and get manuals that should tell you the correct answer
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
You can put it through any cabinet that uses the same 1/4" phone jacks. Make sure that whatever cabinet you choose, it is not so weak that your 150-watt amplifier would blow the speakers, though.
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