What you're looking for is 6W RMS into 6Ω. Or 9W RMS into 4Ω, or 4.5W RMS into 8Ω.
If it does not say RMS, it's probably peak power - which is RMS X 1.414. So if it does not specify RMS, you need to divide the rating by 1.4 to get the right power level. The last thing you want to do is get an...