Two subs or not Two subs?

Are Rel's the best option for high level connections right off of the amp or are their other brands with the same type of connection that work better. Price range $0-$800

I used a single REL for quite some time but always found it directional, no matter where I placed it in the room. Meaning, that I could always detect just where the bass source was located.
I now have a pair of ML Descenti's and the integration with the main 2 channel speakers is great and they are so much more 'tuneable' than REL's. Descenti are not cheap but highly recommended.

You can go to AVS forums in the subwoofer section and look for the ancient subwoofer shootout thread.
Look for the best used sub that $800 can buy. SVS sometimes has scratch and dents for a good price.
For this kind of money you should be able to get amplified Hz just below 20.
Just be warned; I set a $ limit then eventually doubled it.
A quality powered sub will last a lifetime.
No regrets.

A single sub, if located properly AND large enough to excite the OPs 3000 cubic feet would be fine.
My room, larger in all dimensions, and irregularly shaped, is fine with a 12" sub with 'only' 250 watts. It is a HSU Research VTF 2/III.
And even though not rated, it will reproduce the 16hz pedal tone on the Saint Saens symphony #3, 'with organ'.

You can't localize much below 80hz in a normal home space. Like the OK restaurant doing land office business? Location, Location, Location.

The question for me, when I bought, was if I could afford a pair of subs or a single better sub? If I'd had the $$$, the purchase of a pair would have been online.

For music, I always use two subs. They image — they reshape the soundstage as it really was. I have a live Jazz recording I use as a reference — for years, with a single sub, bass player was way back in the soundstage, center stage. Adding the second sub, he moved 10 feet to the left. It was more than just one instrument moving — his relationship to other players and their collective energy was more vibrant, and the archictecture of the nightclub became a real place.