A combo amp contains 1 or more speakers, a preamplier and a power amplifier. The impedance of the speakers is matched to the amplifier to provide the best power transfer.
In a head, the pre and power amplifiers are packaged separately from the speakers. You can select the type of speaker cabinet that you want to use with the head, but you do need to make sure that the impedance and power rating of the head matches the speaker cabinet. A head may have selectable impedance (usually 4, 8 or 16 ohms) while a speaker cabinet usually does not have selectable impedance.
You MAY be able to use a head with the speakers from a combo amp, but you need to make sure that the speakers are not hard-wired to the amplifier (many are, by the way) and that the power and impedance ratings match the head. If you use the head to power your speakers, the amplifier in the combo cabinet will just set there unused.
