How should I wire two 12 inch Power Acoustik Fubar 1800w 800rms dvc 4 ohm subwoofers in a dual chamber sealed box to a American Bass 1000xd amp How can I get max performance with this setup?

It's a borderline setup even in a best case scenario.

I couldn't find a lot of specifics on the amp. It is a 1000W rated amp, but I couldn't find if that is peak or RMS. I also couldn't find out at what impedance that 1000W is being generated.

Even if we assume the "best case" scenario of the amp being 1000W RMS @ 4ohms. That means it SHOULD be capable of 2000W @ 2ohms. If you set those speakers up in parallel they will require 1600W @ 2 ohms. Admittedly, initially that looks good, the amp has more power than the speakers are demanding - but not by much. My rule of thumb is that an amp should be AT LEAST 30% more powerful than the speaker(s) it's connected to. In this setup it is only 25% more powerful, and that's the BEST CASE scenario.

IF that 1000W rating is PEAK, then it's RMS is approximately 500W @ 4 ohms (and therefore about 1000W @ 2 ohms). In a parallel setup, those speakers will need 1600W @ 2 ohms. In that configuration you've UNDERPOWERED the speakers by over 30%. That amp will put out so much distortion it'll kill those speakers in no time.

In conclusion, I wouldn't personally recommend that equipment pairing to a customer. It's too borderline for my liking, it's safety margin isn't large enough.

On the other hand, many installers feel quite confident recommending an equal pairing. "100W of speaker needs 100W of amp". Since you exceed that by 25% in the best case scenario, many would say it's fine.

You'll have to find out the RMS and impedance figures and go from there.