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 overpowered amp and blow your speakers.
So, if the amp is rated peak power instead of RMS, it should be no more than 6.3W into 4Ω, 3.2W into 8Ω, or 4.25W into 6Ω.