subwoofer passive vs. active

Iam looking to add a subwoofer to my system (Alon II by Acarian systems). Which would be better a passive or active (w/built in amplifier)? Could you recommend some models under $ 500 used?
I had an NHT that was not bad but if you do a search for Subs you will find others like ATC,REL,SV and Paradigm. Been awhile since I researched them out.Go for the lowest freq. that you can find. Closer to 20Hz. the better. I have read that Parts Express has a good DIY Kit. I am going to build my own next time or get a REL. Monitor Audio has another one that has been mentioned to me,but I have read negatives by one person who touted the Parts Express sub over it.

As far as Active or Passive it is always better to use Passive with an Outboard amp in my opinion.

One other thought is do not use the Crossover that is in the Receiver if you use one,but regulate it externally.

Hope that helps and opinions may vary!
I've been using an NHT SW3 passive sub for many years. Initially, it was powered by the companion SA3 amp with built in crossover, but the last year and a half I've had much better results using one channel of a Bryston 4B amp with a Paradigm X30 crossover instead.

In most cases the quality of the plate amps installed in powered subs is compromised because of size. Last year the manufacturer of a VERY highly regarded subwoofer told me the 1000 watt amp used in his sub couldn't come close to matching the quality of the Bryston I was using.

It seems that many of the class "D" or "G" amps used in powered subs these days need to have high wattages to compete against a well designed two channel AB amp. My guess is because a good outboard amp won't be restricted by size, it can use a larger transformer, better parts and will therefore be capable of superior current and better sound.

Having an outboard amp and crossover will probably end up occupying more space and costing more, so that needs to be taken into consideration.

Good luck!