Help - Trying to learn & build Amplifier to drive 200 W passive subwoofer

What am I trying to accomplish?
I have an AV receiver which is 7.2. I have one sub woofer connected. I have a passive sub woofer. My AV Receiver subwoofer requires to have a plate amplifier or an amplifier. I am hoping to build a subwoofer amplifier to drive it and make my home theater really 7.2 by adding the second subwoofer.

Subwoofer information
Model: SS-WSB111
Rated 250 Watts
Passive sub woofer

Current Amplifier output information
250 W (at 6 ohms, 80 Hz)

I am hoping to build a 300W subwoofer amplifier. I am not an expert. I am hoping to help from experts to learn and enjoy the journey. My budget is $200 - $250. Lower expense and higher quality is I am hoping for. Could you help me?
 
Building "just one" of most anything is more expensive than buying it ready made, unless it's something exotic.

I bet a ready made plate amp will be your best solution.
Thank you for the message!

I am here hoping for do it myself and learn. A ready made is not a diy. I consider at least a kit to be a DIY. ;) I hope there will be an expert out there who is willing to share knowledge and expertise.