CONNECTING A SUB WHEN THERE IS NO SUB-OUT CONNECTION

Depending upon subwoofer placement, it may prove easier to just run a jumper from the (+) and (-) terminals of each speaker to the respective speaker-level inputs of the subwoofer. The subwoofer has a high impedance input (e.g. 100k+ ohms), so the current running through the jumpers is tiny.

Ideally, the connection could be simplified by using the extra L-R outputs of the BAT Rex preamp to connect the Martin Logan transmitter ($100 USD) that sends a signal to the sub(s) wirelessly. The only hitch is the transmitter has single ended (RCA) inputs only whereas this preamp has only balanced XLR outputs, so appropriate cables would be necessary.

@patrickalston , let me edit this post. I overlooked that you have mono blocks. [hence the * in the post below].

I don’t know if it is different for your subwoofer, but for REL, i know you cannot hook up 2 monoblocks to a single subwoofer.  Doing so creates a serious risk of shorting out the amps.  REL recommends one subwoofer per monoblock.  So the advice above and what I’ve written below still is correct, but you would chose just right or left channel to connect to a single subwoofer or transmitter. 

I am not sure if the same applies if you are connecting the subwoofer to the speaker’s binding posts, but I’d check with your dealer or Martin Logan before I, in essence, wired those speakers to each other via the subwoofer. 

Here is a variation on @ted_b ’s suggestion above: the REL Airship II is marketed as being able to work with other brands subwoofers and it makes sense to me that it should. 

https://upscaleaudio.com/products/rel-acoustics-airship-wireless-transmitter

Amazon with its lovely 30-day return policy also used to sell these — i bought one some months ago to give me greater flexibility in locating my REL Carbon Special, but at least when i just check now, they show them as out of stock.

You would attach the transmitter module to your amp’s speaker output* or to your speaker’s input post* and then connect the receiver module via RCA interconnects of whatever length you’d need to the speaker-level inputs on your subwoofer. (If you got a 2d subwoofer, you’d need another Airship II.)

You will likely also need to ground the Airship to the chassis of the mono block or float the ground wire and run an interconnect from a spare output in your preamp into the Airship (XLR on pre-amp end and RCA on the Airship II end.)

Is that more attractive than going from XLR connectors at your preamp to single ended RCA at the Martin Logan transmitter — which your subwoofer may have come with?  

-Kirk

Oh, and @patrickalston to your original question, 

Is connecting the sub as simple as using a spade-to-banana connector between the amp and the sub?

I think the answer to that is yes. Spade end on your amp’s speaker binding posts (or on your speaker’s binding posts, if they take spades) and (i think) standard speaker banana plugs into the speaker-level input (one channel only) on the back of the subwoofer. The “micro-site for your subwoofer https://products.martinlogan.com/dynamo/ says that the speaker-level inputs require bananas.  (If your speaker’s binding posts have available bananas plug inputs, you could just go banana-to-banana into one of the subwoofer speaker-level input, i think.)

I didn't take the time to look at your preamp, amp, or sub but I've definitely run into similar situations in the past. 

I've done the splitter cables coming out of the preamp to amps & subs. This doesn't work well for all components & impedances. 

I've done the wireless connection to the sub..which still needs the signal plugged into the transponder that wirelessly sends the signal to the receiving device plugged into the sub. This also involves plugging extra devices in somewhere. 

You can wire directly from the amps speaker outputs to sub. 

I have found something that I prefer over these others. SVS makes a line level device that wires/connects to the left & right outputs on the power amp. And then allows you to run rca cables from this device to your sub. No balanced outputs though. 

It's a sub $100 device that works. 

Get that second sub before you do damage to your amps. Then connect each sub’s high level input to its respective amplifier. I had this dilemma with my REL Classic 98 and pair of Quad 303 mono amps.  Fortunately my preamp has both balanced and unbalanced outputs, so I run the amps off the XLRs and use the RCAs to feed the sub with a Y connector.