Yes, they do need to match at least closely. Ohms are a measurement of impedance, and while it can get tricky when you deal with changing loads, in practice it's pretty simple, just plug 4 ohm outputs into 4 ohm cabinets, and 8 ohm outs into 8 ohm cabs. For better information, google, "Ohm's Law" to learn how this works. In the meantime, match your amp's output to the speakers, or suffer from bad sound at least, and blown transformers at worst.