There’s something so alluring about tube amplifiers. My first purchase from Woo Audio was the gorgeous WA7 Fireflies. They were fantastically articulate, transparent, and quiet. Over the course of a few shows, I had a chance to listen to their WA5-LE which left an imprint on my mind. It was paired with upgraded tubes, the Abyss headphones, and JPS Superconductor cables and was memorable due to its overtly seductive sound signature. It really sang to my core. Tubes aren’t for everyone as they typically come at the cost of some resolution, depth, and defined sonic outlines but most find the euphoric golden bloom to be too much to pass up.

When Woo Audio announced their new WA33 amplifier, I was at their booth first thing in the morning at the last SoCal CanJam. The early bird catches the worm and I was able to spend plenty of uninterrupted time with this beauty. It sounded quite a bit different from the WA5-LE as it more tonally balanced and quieter. It worked. It made the Focal Utopia and Abyss Phi headphones sing and kept me firmly in the listening chair.

I knew I needed more alone time with this amp and asked Woo Audio for a review piece. Luckily my upgraded Abyss Phi came just in time for this review.


  • Source: Chord DAVE with SOtM Trifecta
  • Interconnects: Audience Au24 SX RCA
  • Power cords: High Fidelity Cables CT-1 Ultimate
  • Headphone cables: Double Helix Cables Complement4 and Danacable Lazuli Reference
  • Headphones
    • Abyss Phi
    • HiFiMan HE-1000 V2
    • Mr. Speakers Aeon Flow Open
    • Sennheiser HD-6XX (Massdrop)
    • Meze 99 Classics
    • 1More Triple Driver Over-ear

Build & Features

Price: $7,999 (base). Comes in black or silver.


  • 4 x 2A3 power tubes in matched quad
  • 4 x 6C45 driver tubes in matched quad
  • 1 x 5U4G rectifier tubes


  • 2 x XLR inputs
  • 1 x RCA input
  • 1 x XLR preamp out

The build quality of this amplifier was immediately apparent. Installation of the tubes was a breeze. Just make sure you align the notches with the 5U4G and follow the instructions provided with the amplifier. Btw, this amplifier is pretty heavy (50 lbs). Handle with care.

The WA33 is a fully balanced, direct-heated triode, Class A, output-transformer design. Jack Wu of Woo Audio was describing their Internal Coupling Architecture (ICA) technology, which was also used in their much acclaimed WES electrostatic amplifier. Instead of using an umbilical cord with an external power supply, The signal path is shortened by connecting the amplifier and power supply internally. This eliminates any potential RFI/EMI noise from additional components. As with all things audio, the lower the noise, the better. A quality power topology will elevate a component to new heights and a proper power supply design is crucial for optimal sound quality. Nice move.

The WA33 pushes about 10 watts and could also be used to drive speakers directly. Unfortunately, I didn’t have this custom option with the review units. Would’ve been interesting to drive them with the Omega CAMs.

There’s something satisfying about hitting the orange power button and turning the knobs on this amplifier. Fantastic build quality commensurate with the price.


I experimented with a few dampeners, specifically the UltraSonic Rx from Herbie’s Audio Lab. These dampeners tights up the image and provide more speed at the expense of warmth and body. Imaging is also slightly improved. Placing a varying amount of dampeners affected the output almost linearly. I personally preferred not having the dampeners at all but some will prefer the speed and clarity of having them on.

Impedance and Level

With the Aeon Flow Open, I get more shine and clarity by setting the Level and Impedance to H but a smoother sound when Impedance is set to L. With the Abyss Phi, I had Impedance and Level on H which provided a very vivid and punchy sound. Having them set on L was smoother but less engaging to my ears. Having Level set to high provides more detail, a larger soundstage, more dynamics, but is a bit leaner.

These helpful knobs allow you to tune a wide range of headphones of varying impedances and sensitivities. There’s no concrete way of setting these, try all permutations and stick to what sounds best with your system. After testing with over five headphones, I find different settings could work on the same headphone for different moods. A welcomed feature indeed.

Balanced or Unbalanced?

Luckily I had two Lazuli Reference headphone cables here, one 1/4″ and one 4-pin XLR for direct comparisons. I would use only the balanced connection if possible because it’s:

    • immediately quieter and more holographic.


    • has much better layering and separation. No smearing or bleeding whatsoever.
    • more dynamically rhymic.


    • super crisp, clean, and accurate.


    • much more punchy without being in your face and aggressive.


  • sounds closer to what I hear from my 2-channel setup.

Basically, recordings sound completely different (better) in balanced mode. There’s just more music, depth, body, air, and control. The single-ended connection doesn’t even come close.