CONNECTING A SUB WHEN THERE IS NO SUB-OUT CONNECTION

Equipment

BAT Rex Preamp

BAT VK-120 mono blocks (i.e., two VK-60s converted from stereo to mono) and

Martin-Logan Dynamo 10 Subwoofer (and, yes, at some point in the near future I plan to add a second sub). 

I hope this works - Here's a link to rear panels:  https://drive.google.com/file/d/127__W75PZ6Nx679IzPJCNQZTmzf6OScG/view?usp=sharing

The best way to connect a sub is from the amps speaker terminals directly to the 'high level input' of the sub. 

^^ this ^^

  Patrick , looking at the back of your sub I’d use the speaker level inputs as mentioned above. You would need to run a pair of cables from the backs of your amps . The only balanced input is LFE but you’re not running a home theater setup. With all due respect you have a fantastic system way above my income bracket and would be well served by adding a pair of subs that match the quality of your system. I’m ASSUMING that you are just experimenting with a sub on hand to see what it sounds like. If that is the case and you are connecting at the speaker level and not from the pre you could probably get away with building a set of cables yourself. Respectfully, Mike B. 

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