Does AMP AC/DC adapter going through a step down increase in amps per voltages (ECIG)
If the "step down" is a linear regulator you'll get slightly less than 750mA out of it, at most. 

If the "step down" is a switching regulator, you'll get somewhat less than the input power, say at least 75%. Since your input power is 12V * 750mA = 9W, you could probably count on getting maybe 7W out of it. So you could power the first two indefinitely (if the specs are correct) but not the last two. 

Of course the output voltage has to be set appropriately for the load resistance before connecting the load. 

Edit: so that means that with an output voltage of 6V your switching regulator would be able to supply 1.5A minus the losses in the regulator. Even if you reduce the voltage below 4.5V you should not attempt to draw more than the 2A limit of the regulator.