500 watts will be just noticeably louder then your 300 watt amp. You would notice a difference with 750 watts, your sub would be a bit over xmax but can handle it. To get that power in a plate amp you'd have to go with the Dayton SA1000.
I am also looking for a plate amp for a new HT sub project. I am looking far a 500w. I do have some concerns though from review I have read on some. The Dayton 500 seems to have a humming issue and sleep pop. I also believe that the Bash on The Parts express sight has a sleep mode pop issue but may be wrong on this. Let me know from everyones experience what is a good one. Also any reviews on the Yung. I just don't want to buy a new amp and have these type of issues