High level connection from Esoteric integrated to Sonus Faber subwoofer

@scottwsmith which Esoteric amp? Is it fully balanced? If so, i think you are not supposed to connect the black wire to the negative speaker post on the amp ; that is not a ground on a fully balanced designed ; as far as I understand things — and this is based entirely on what REL says to do and a bit from talking to an amp designer — you are risking shorting out either your subwoofer, or your amp or both. And apologies if this is already obvious to you. 

The amp is a E-03A balanced class A.  The subwoofer is a Sonus Faber Gravis II and yes it uses a neutrik connector.  It is locked.  I did find one article on the web that said to connect the red and yellow wires together, instead of connecting them to the opposite speaker posts.  Need to check that out again.

I believe the preamp section of this amp is balanced, but not the power amp section.  As a test, I connected the red/yellow wires to the respective speaker B positive posts and the black wire to the amp's ground post.  Still no sound from the sub when I select speaker B output on the amp.  It really has me stumped.  This should be easy.

I would consult Esoteric directly or a dealer since your integrated is dual mono and requires a specific connection to prevent shorting.  If you’re feeling lucky this is what Gemini says:

### ⚠️ Critical Warning for Balanced Amplifiers
Because your Esoteric (likely referring to the **F-03A** or a similar Esoteric Class A model) is a **fully balanced, dual-monaural differential amplifier**, its negative (-) speaker terminals are *not* at ground potential. Instead, they carry an active inverted signal.
If you connect the black wire of a high-level Speakon cable to a negative speaker terminal on a fully balanced amp, you will short the amplifier's negative output stage directly to ground. **This can cause catastrophic damage to the amplifier.**
### The Correct Speakon Wiring Protocol
To safely connect the Sonus Faber Gravis II (which uses the standard Neutrik Speakon wiring protocol) to a balanced amplifier, you must leave the negative speaker terminals completely alone and utilize a true chassis ground.
Here is the exact wiring configuration you need to use:
 * **Red Wire (Right Channel):** Connect to the **Right Positive (+)** speaker terminal on the Esoteric.
 * **Yellow Wire (Left Channel):** Connect to the **Left Positive (+)** speaker terminal on the Esoteric.
 * **Black Wire (Ground):** Connect to a **Chassis Ground** on the amplifier. Do *not* connect this to any speaker terminal.
### Step-by-Step Connection Guide
 1. **Power Down:** Ensure both the Esoteric amplifier and the Sonus Faber Gravis II are completely powered off and unplugged from the wall to prevent accidental shorts during installation.
 2. **Connect the Positive Channels:** Loosen the Right (+) and Left (+) speaker binding posts on the Esoteric. Slide the bare wire or spade of the **Red** wire into the Right (+) post and the **Yellow** wire into the Left (+) post, right alongside your main speaker cables. Tighten the posts securely.
 3. **Locate a Ground Point:** Because your Esoteric integrated features a built-in phono stage, the absolute safest and easiest grounding point is the **Phono Ground Post** located on the rear panel. If the phono ground is occupied or inaccessible, an unpainted chassis screw on the rear panel serves as a perfect alternative.
 4. **Connect the Ground:** Attach the **Black** wire securely to that Phono Ground Post or your chosen chassis screw.
 5. **Engage the Speakon:** Plug the Neutrik Speakon connector into the "High Level" input on the rear panel of the Gravis II. Twist the connector clockwise until you feel and hear it click into its locked position.
 6. **Power Up and Tune:** Plug your equipment back into the mains. Power on the amplifier first, followed by the subwoofer. You can now use the independent High-Level volume and crossover dials on the rear of the Gravis II to perfectly dial in the blend with your main speakers.
 

This from the web: 

Yes, the Esoteric F-03A features a fully balanced, dual-monaural preamplifier section. It utilizes Esoteric's proprietary Quad Volume Control System (QVCS), which incorporates four precision switched-resistor networks to independently control the positive and negative phases of the left and right channels. 

theaudiosolutions.com +2

While its preamplifier stage is fully balanced, its power amplifier stage is not bridged balanced; rather, it provides a high-quality, pure Class-A single-ended topology.