Choosing a subwoofer

What's your budget?

Look into Sundown SA series, FU Audio 750 series, Resilient Sounds Gold series, Pride Audio HP series, ect ....
Any one of those with a simple 1k rms amp will do great ....

Focus on enclosure first. How much space you're willing to give up. Sealed or ported. If you are not wanting to build your own enclosure or know anyone who will build one for you and plan to buy one on line I'd suggest sealed over one of the pre fab on line ported enclosures.
Once you know how much air space and type of enclosure,budget,what you're expecting out of it, spl or sq or somewhere in between, plus type of music you listen to, people will be able to steer you in the right direction.
Going stand alone will get you better results than going with a powered sub.

if going with a 250 to 300 watt rms sub I'd be looking at 500/600 watt rms sub amps. 500 to 750 rms sub 800 to 1.2k ish amps. Best to have some head room especially if going with a seald enclosure. Sealed isn't very efficient.