Assuming the speaker in the combo is an 8ohm, you'll need a 8ohm extension cabinet. Ohm's law says that 8ohms+8ohms=4ohms since you'd be connecting the speakers in parallel. Really, if you're getting into bigger rigs you need to look up ohms law to understand how things go together.
You need to find out what resistance (ohms) the combo amp's speaker is. If it's 4 ohms and the amplifier can't go lower than 4 then you can't add another cab. If it's 8ohms (which most are) then you're good to add another 8ohm cabinet.
What the manual means by output rating is the amplifier section of the combo. It can run down to 4ohms without overheating and destroying itself. The less resistance the amplifier faces, the higher wattage it can put out, so if it has a 8ohm speaker connected to it, it puts out half it's rated power. If you have 2 8ohm speakers in parallel, the resistance is effectively halved so it can put out it's maximum power since the amplifier section is running at it's optimal load.
Best bet is to find a good 8ohm cabinet if my assumptions are correct about the speaker. Oh, and study up on ohm's law for sure, it'll help you a lot in the long run.
You need to find out what resistance (ohms) the combo amp's speaker is. If it's 4 ohms and the amplifier can't go lower than 4 then you can't add another cab. If it's 8ohms (which most are) then you're good to add another 8ohm cabinet.
What the manual means by output rating is the amplifier section of the combo. It can run down to 4ohms without overheating and destroying itself. The less resistance the amplifier faces, the higher wattage it can put out, so if it has a 8ohm speaker connected to it, it puts out half it's rated power. If you have 2 8ohm speakers in parallel, the resistance is effectively halved so it can put out it's maximum power since the amplifier section is running at it's optimal load.
Best bet is to find a good 8ohm cabinet if my assumptions are correct about the speaker. Oh, and study up on ohm's law for sure, it'll help you a lot in the long run.
