What do a cabs and heads do?

well, in a nutshell, a head is what tells the speaker to make the sounds your inputting, and EQ and effects and stuff, but a combo has a head built in. cabs dont have a head built in, but they have input jacks, so you're basically hooking up straight to the speaker, with no EQ or anything, unless you buy a head.

 

You will need a head before the speaker though.

From the last post It seemed (unintentionally im sure) that you could bass->cab, you would just lose eq and such.

 

Check the "sticky" threads at the top of this page. There's an amps FAQ page that has answers to these types of questions.

 

Heads decide what amps to match up with cabs. Then heads talk to their wives in hopes of convincing them to purchase said amps-n-cabs.

 

It.s sort of like a combo amp sawed in two. With the amp/cab rig you have the amplifier in it's own separate case. Think of an Ampeg SVT or Peavey Classic 400. Something like that. Having it in a separate case makes it light and easy to carry. The speakers are also mounted in their own cabinet. You've seen them on MTV probably. A couple of examples would be the Ampeg 8x10 or the Mesa Diesel 2x15. Same theory behind that. Lighter and more easy to carry.

Then you have the combo amp rig. Here the amplifier and the speakers are all mounted together in one giant box. Instead of two light and easy to move pieces, all of that weight is put together into one heavy, cumbersome package.

Another difference in the two types of rigs is what is called the F.R. or fartage ratio. The formula for computing the F.R. is: (W) x (UNV>TOF) = (FR) or, weight multiplied by the percent of unusable volume over the threshold of farting equals the fartage ratio. You can do the math. In all but a few rare examples, the combo amps have the highest fartage ratios.

You may want to read the very informative and entertaining "Pros and Cons of Combo Amps" megathread. Just do a search on "combo amps".