Ultimate 50x50cm cube sub?

Hello, what would you consider ultimate passive diy sub for home (medium room) with outside size max 50x50cm. Prefferably for movies and music alike. Regarding "ultimate", lets say price tag for speaker is 800usd (no extreme stuff made of vibranium plated antimater).
 
It depends on what kind of sub you want, just a boom or a musical sub. In any way i would go sealed, or TL if sealed is impossible (but your size limit blocks that).

Lowest high quality high volume for that space will be an Dayton UM12-22 in a 90L sealed i think. A bit more musical could be an SB34NRX75-6 in the same cabinet. A bit less deep (but still F6 below 30Hz) or loud (but still louder than you would use in a small space) but way more refined and musical.