300 watts is the rms rating of the 485. 600 watts peak. But the 485 can take a lot more power and loves it!!
jmuck is correct, you are always more likely to blow a speaker with too little power. Because you wind up taxing the amp and over compensating the gains so you get a clipped signal which is what blows speakers.
With more power, you do not wind up turning the gains up past their "unity" setting so you gain headroom which results in more clean power. That amp should be good power though. Enough to get good clean sound and no distortion.
You want to look at a system as a whole and start at the beginning. A speaker only reproduces what it is fed. It is at the end of the line. So distortion comes from the beginning back.
Source material. CD or MP3 player? Music highly compressed? EQ settings on the MP3 player? Volume up on the MP3 player? Make sure that the mp3 player has no eq settings on
Head unit. Make sure that is flat as well. since you have a 420, you donít need any additional eq settings.
WS-420 FLAT for tuning. Amp bass boost off.
Here is a step by step tuning guide I wrote
Take a look at that and follow it.
Also, since you have the HSE box and a WS-420. You have DUAL line drivers. Which is probably where you are getting your issues from. You are boosting you voltage a TON to the amps.
With this much voltage going into the amps. The amp gains will probably not even be up past 1/4. If they are. Your gains are way too high and this is why you are getting distortion. I would bet you cannot get the head unit volume much past 50%. If this is the case, gains are way too high.
Always remember that a gain does not make the amp put out more power. That amp will do 300x2 and that is it. Once that gain is at itís unity setting and optimized, it puts out 300x2. Anything higher on the gain, does not result in more power only more distortion and less control.