Need 250 to 350W into 4R

I need about 250W to 350W in 4R mono to power my subwoofer. I am running it with a P3A (about 120W into 4R) and it sounds pretty good, but i need more power when i have built my Linkwitz Transform circuit to take the sub down to 28Hz (-3 dB).

Since it is just for Bass duty, i'm not that concerned about getting 0,000000000001% THD or perfect audiophile sound quality (although i do want that for the rest of my system obviously) and would like suggestions of a DIY amp which is quite easy to build, concidering that my abilities are at the P3A level.

Any suggestions would be great. Thanks all.

Gareth