Ratings for drivers for hi-fi/audio use seem to be a little, erm, over-stated. They may say it will handle 150-200w, but I'm pretty sure you could fry it with a 100w guitar amp, especially if you start adding some dirt to the sound. So 5-20w should be fine. I'd suggest looking at the efficiency figure, though. Hi-fi/audio drivers are often in the low 90db range, whereas guitar speakers are typically around 100db. 3db less efficiency requires double the input power to generate the same loudness, so it's important as the difference can be large.