The speakers should be 100w each in a 2x12, in a 4x12 they should be 50w speakers in a 1x12 you need 200w speaker. The confusion on the internet is that some people are afraid to say they don't have the cash to buy new equipment but would like to use the amp they have to power a cabinet they came across for short cash. The quick answer is, yes you can, just don't turn it up and most people don't anyways. How many times do you run an amp at max volume. I can across this web site that clears it all up. It's a little techy but if you read it 2 or 4 times in comes clear what you can run:(website)the short version is this: 100w head; 1x12 cabinet 200w speaker; 100w head 2x12 cabinet= 100w (each) speakers; 100w head 4x12= 50w(each) speakers. If you have a 50 watt head do the math: 50w head 1x12=100w speaker;50w head 2x12 cabinet= 50w (each)speakers; 50w head 4x12 cabinet= 25w (each) speakers, yet Warehouse is a good source for speakers and another good source with wiring diagrams and info with speakers is Orange County Speaker. I have never seen such confusion on one subject in my entire life. People mix options with facts which is or can be two different worlds.
Next up is you need to know if the cabinet speakers are wired in series(one after another) or parallel (side by side)For people trying to understand parallel imagine a ladder the vertical rungs of the ladder is the power and the rungs that you step on are the speakers, series is easy because it is wired one after the other in a row. Opening up the back panel will clear this mystery up for you. Don't measure the resistance of the line into the cabinet and make an educated guess because it could be incorrect, there are 16ohm speakers out there and if you guess that it's 8ohm speakers in series then you could be wrong, it could be 16ohm speakers in parallel. Measuring the 1/4" connector on the back will tell you what the cabinet is, lets say you measure 10 ohms this would be a 8ohm cabinet the other 2ohms are because you can't have a "perfect connection" when measuring the ohms and it will be slightly higher than expected but not always. Speakers wired in series you add the ohms together, like 4ohm speakers in a 4x12 cabinet = 16ohms at your amp output selector. Most amp heads have 4, 8 and 16ohms positions to chose from and if yours doesn't sell it and buy one that does it just keeps the available option at a maximum. You can measure the connection and determine what ohm setting to put the amp on but not to determine what speakers you have. Hope I don't add to the confusion and this clears it up or brings it down to earth for ya. Here is another good explanation of the confusion: Cleveland Institute of Electronics