You can use 20W rated speakers. You run the risk of damaging them with an 80 source if you play loudly. If you don't play loudly, then you might be OK. The bigger question is what it the impedance of the speakers (4 ohm, 8 ohm, etc) and what is the minimum impedance the amp supports. You might be OK with wattage, but might damage your amp with an impedance mismatch.