Sub woofer naive question day

Get a REL sub. It connects to the speaker taps with a Neutrik Speakon cable. No need for a specific sub tap.

Speaker level inputs on the subwoofer would be the best way to go.  A lot of good subwoofers have speaker level inputs.

"Sub out" indicates a low-frequency output, intended to be fed directly to a powered subwoofer.

If I can ask a related question, when an amp has a “sub out” does that connection normally cover the entire frequency spectrum or just the lower end? Maybe there is no “usually”?

When it comes to sub(s) two is better than one and one is far better than none. REL has lot's of videos as doe Youtube.

SVS now sells an adapter to drive any subwoofer that has an RCA input, using a speaker-level connection. I have not tried this product, but it's inexpensive and comes with a home trial period.

Thanks everyone. 
The responses are more helpful than y’all will ever know. 

Despite what they call it “20Hz@94dB 40Hz@100dB” is a dramatic -6B roll off.

Does their app allow you to adjust more gain at 20Hz?

It looks like the sub10 is designed to work best with Burchardts's active speakers. As Deep_333 states, you would connect your amps pre out to the sub10's input and then connect the sub10's output to one of Buchardt's active speakers input.

You would run the amp's RCA "PRE OUT" to the subwoofer's RCA input.

Currently Spendor D9. But will be getting some Burchardt e50’s and their sub10. 
I believe the answer to the question is yes. 
However this will answer the question

Guess I cannot post a screen shot. 
In amongst the pictures is a schematic of electrical layout. 
https://buchardtaudio.com/collections/active-speakers/products/sub10

Does the subwoofer have speaker level connections?  What type of speakers are you using?