OK,AED, here's the scoop...
That Marshall head will need to "see" the correct "load" (the Ohms part) to operate correctly. You need to understand, first and foremost, that running ANY tube amp into the incorrect load will ruin the output transformer and prematurely eat up the power tubes. Not good.Lots of cash. There is a selector on the rear panel of the amp that lets you "tell" the amp your load is, speaker-wise. So if you are simply connecting a 1960 cab to the amp, you would set that selector to "16 ohms", if that's what the cabinet is rated at . At least that is what that cab USED to be rated at in the good old days, but that may have changed. Some Marshalls have a selector or different jacks on the back to operate them at different loads, like 8 or 4 ohms. Bottom line is, look on the back of the cab, and whatever load it says you are operating at, you set the amps' selector for that.....IF YOU ARE ONLY USING THAT ONE CABINET!!!!!!!!
Things get dicey when you hook up another cabinet! Because now you are going to set the amps' selector at whatever the TOTAL LOAD is with 2 (or more) cabinets connected to it. This will require a bit of simple math,OK? If you are hooking up (2) 1960s, and they are rated at 16 ohms each, you are presenting an 8-ohm load to the amp.This is because whenever you "parallel" 2 cabs together, you divide the # of cabs into the # of Ohms 1 cab is, so 16 Ohms divided by 2 cabs= 8 Ohms. OK?
If the cabs are each rated at 8 Ohms, 2 of them will yield a 4-Ohm load, dig?
Now, IF you are using a 2X12" cab, it should be rated at the same load that the 4X12" cab is rated at, OHMS-WISE....SCREW THE POWER RATING AT THIS POINT! If the 2X12 cab is 8 Ohms, and you have it connected to a 16-Ohm cab, you will be essentially delivering TWICE the power to the 2X12 than the 4X12, because it is presenting half the resistance to the amp head!!! And the net load on the amp is whacked, like 12 Ohms, divided by 2=6...you got a 6-ohm position on the amps' selector? NO!
So, make sure that the 2X12 cab is rated at the same load as the other cab, just for the sake of simplicity. As far as POWER ratings are concerned, most modern 30-watt speakers will do fine. I personally have stayed clear of the newer Celestions because they are made in China, but that's my opinion. I like Eminence USA speakers (Legends, Redcoats, Tonkers,etc) because they are tougher, a bit louder, and, IMHO, sound better. They also provide a much broader assortment of available tonal "voicings". It's interesting to note that the newer Marshall 1960A/B cabs that I have seen have "Eminence Made in UK" speakers in them!
It is truly a global economy we live in, ain't it?

Jim "I Like My Women Stacked and My Marshalls Slanted" Mooney

thanks jim,
I was planning on peicing together the 2 x 12" cab. so I will make sure I buy the correct speakers so that the ohm load matches the 1960a. And I believe the 1960a is now 8 ohms but capable of running a few different ways. I will take a look at how I should wire a 2 x 12" cab. together but if they run parallel they would need to be 16 ohm speakers. Correct? 16 ohms / the 2 is 8 ohms all together. Then the two cabinets together would be run at 4 ohms because the two at 8 ohm / 2 is 4 ohms. Correct?

So taking into accout the 1960a is switchable do I want to try and make a configuration that runs at 4 ohms or 16 ohms? Not that I could do 16 ohm because you'd need 32 ohm cabs. But say 4 or 8 ohms. More being 8 ohms of resistance less at 4 ohms. Is one better for the speakers to run at or the head? Thanks.

Alright, AED, we are going to say that the 1960 IS 8 Ohms...but by all means, verify that before you proceed, OK?
Yes, then you'll want to make sure that the 2X12" cab is 8 Ohms, too. Together, these will yield a 4-Ohm load on the amp. And that is cool.
Now, how to make the 2X12" an 8-Ohm cabinet...
You can take (2) 16-Ohm speakers and parallel-wire them together.BTW, that's taking "+" to "+", "-" to "-". This will yield an 8-Ohm load for your cabinet. I definitely do recemmend the Eminence line-up for that cabinet, and you can order most of their models in a 16-Ohm version. A great source of raw speakers (both Celestion and Eminence) is AVATAR Speakers here in the States. Google them and go to their website.
they also offer great deals on cabinets, but if your budget is tight, you can sure build a box yourself and save a bunch. Also, having a different type of cab, say, an open-back design with a pair of Swamp Thangs, will give you a wider variety of available tones, with or without the 1960...Good Luck!

Jim "I told Bode Miller to Chug the 12-Pack AFTER the Race!" Mooney

so i was looking at the swamp thangs and they are rated at 150watt rms. so they would be a 300watt cab. My question is. Back when my bass player was buying his bass cab and head a guy told him he should try and match wattages. Like if his head was 400watts his cabinet should be around 400, like 450 or 500 I guess. Is this because you would use more of the cones response like the cone would move back and forth more and give a more dynamic sound? So if my head is 100watts wouldn't 300watts be exsesive? And say I'd be only using about 1/3 of the cone if how im think is true. So I should look for 2 75watts. Does wattage go up the higher you turn the volume up? So at 5 you would be producing about 50 watts with a 100 watt head? Or is that not true?

Okay, sorry, I just couldn't stay out of this one ...

Mixing cabinets in this way really isn't a great idea. Let me break it down for you:

Let's say you take a 4x12 cabinet which is 8ohms and a 2x12 cabinet which is 8ohms, and run them parallel into your amp. For simplicity's sake, let's say you are driving exactly 100 watts into the total load. Each of those cabinets is going to share that load equally, 50 watts to each cabinet. Each 12er in the 4x12 cab is going to get 12.5 watts, and each 12er in the 2x12 cab is going to get 25 watts. This will be a major problem. The two 12s will always be much hotter than the four 12s, and you'll have one hell of a time getting a decent sound out of the rig at any volume, because what sounds good through the 2x12 will sound like crap through the 4x12 and vice-versa. Also, if you do happen to get lucky and find a decent blended sound, you'll totally lose it with any small change in volume, including your guitar dynamics.

If you are dead-set on a 2x12 cabinet, all is not lost, especially since your cabinet is 8ohms. Take a look inside your 4x12 cab. I assume it will have four 8ohm drivers wired in a series/parallel combination. Whatever drivers are in there, buy two more of exactly the same driver and put them in a 2x12 box. You now have six 8ohm 12s. Wire them into three series pairs (16 ohms per pair), then wire the three pairs in parallel for a 5.33 ohm total load. Set the output transformer on your amp to 4 ohms and you're totally safe and good to go. DO NOT use different 8ohm drivers! They will have different Fs, Qts, Xmax, et cetera, and you again could be at risk of having an unbalanced, unruly sound. Mixing two twelves in a single cabinet can sometimes have great results, but mixing drivers in a group will usually be terrible.

well i just looked and the whole setup is actually at 16 ohms. i had it all setup correctly but at 16 ohms i dont know what kinda 2 x 12" setup would work so I might just hold off till i have money for another 4 x 12". thanks for the replies.