Review: Universal Audio OX Amp Top Box

The Universal Audio OX Amp Top Box is a brilliantly engineered device with many application possibilities.
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Amplifier attenuators have been around for decades, and modern, reactive-load devices have become standard items for players who like the sound of their amp running at full bore but need to reduce the wattage that hits the speakers to keep the volume in check.

True to form, Universal Audio’s OX Amp Top Box ($1,299 street) is designed to let you run your tube amp (up to 150 watts) at its optimal volume setting, while letting you control the output to the speakers with a 6-position Volume switch. However, good as it is at preserving the tone and feel of an amp while significantly lowering the volume, OX is far more than a mere reactive load box. It also routes some of the attenuated signal into a digital section that—when the software app for Mac or iPad is enabled—provides 100 “Rigs” (i.e. preset combinations of various speakers, mics, and effects), that can be modified to create your own custom rigs by selecting from a menu that includes 17 speaker cabinets, eight close mics and six room mics, as well as studio-grade compression, delay, EQ, and plate reverb.

Note that the “modeled” sounds are only available from OX’s dual ¼" balanced line-outs, S/PDIF digital outs (RCA and optical TOSLINK), and headphone out. In other words, they do not affect the attenuated signal that’s feeding a standard speaker cabinet.

OX’s front-panel controls also include separate Volumes for the line and headphone outs, a six-position Rig selector (which is user configurable via the software app), and a Room knob that varies the amount of “studio room ambience”—turn it up for an airier sound, down for a drier response. Room only affects the line, digital, or phone outs, and its settings can also be edited and stored via the app.

"The OX app for Mac and iPad displays
 your selected speaker cabinet and allows
 you to choose from a large selection of
 microphones, adjust their positions, alter
 the damping of the room,
 and add studio-quality compression,
 delay, EQ, and reverb to create your own
 custom Rigs."

"The OX app for Mac and iPad displays  your selected speaker cabinet and allows  you to choose from a large selection of  microphones, adjust their positions, alter  the damping of the room,  and add studio-quality compression,  delay, EQ, and reverb to create your own  custom Rigs."

You can use OX without a computer, of course, but the app is necessary if you want to deep dive into all of the pre-configured Rigs and/or edit those presets to create and save your own custom Rigs. And since the app is such a big part of the user interface, UA thoughtfully equipped OX with its own built-in Wi-Fi network, which—once the app is downloaded onto your Mac or iPad and a password established—provides reliable communication between OX and your chosen device, regardless of Internet availability. The only problem I had was that OX stopped passing signal twice during these tests and had to be switched off and on again to restore operation. The app was not running at either time, so the cause didn’t seem to be Wi-Fi related.

The hardware side of OX is brilliantly executed. The unit features a heavy-gauge aluminum housing with plenty of vent openings, and the front panel looks retro with its two-tone color scheme, large knobs, wood trim, and jeweled indicator lamp. The in-line 12VDC power supply connects to main unit via locking XLR jack, and extra-tall rubber feet provide plenty of clearance for the amp’s handle when placing OX on top of a head or combo. Nice! There’s a footswitch jack on the rear panel that the manual describes as “not functional.” However, by plugging in a momentary footswitch (not included) OX turns into a looper of sorts: It records, plays, and stops, but won’t store anything. Heck, I wouldn’t mind if it just let you toggle the Rig selector via footswitch.


Even before I started controlling OX with the app, it was fun to hear my amps though the six “factory” selections of miked 4x12, 2x12, 1x12, and 4x10 cabinets available on the Rig dial. The really cool thing is that my vintage Marshalls (’66 JTM 50, ’69 PA 20, and ’72 JMP 50), as well as a Fender Deluxe Reverb, Mesa Mark 5:25, Sound City SC30, and Vox AC10 all maintained their distinct sonic and dynamic signatures when driving into OX at all sorts of different volume and gain settings. It’s literally like taking your amplifier collection into a great-sounding room (which can sound two different ways by using the DAMP function), with awesome mics and hearing them as perhaps you never have. Of course, the app lets you switch speaker cabinets, use different mics and adjust their positions, layer on superb sounding effects, and save these custom sounds in the app and/or to the Rig selector for live or recording situations where you don’t want to be toting a computer or tablet. Pack your fave amps and OX Top Box, and now you’ve got studio-quality tones at your fingertips with zero hassle. And since all of the outputs are available simultaneously, OX can readily be implemented into a live sound setup, where you’d run your amp though standard cabinets onstage, send a direct feed from the line-outs to a monitor system or the FOH mixer, and maybe use the digital outs to feed a DAW or other device. If you’ve ever lusted for a 3D soundstage when playing larger rooms, OX can certainly help get you there. OX is a brilliantly engineered device with so many application possibilities, but even if you simply use it to enjoy your tube amps in astonishingly realistic studio environments, there’s a mighty good case for owing one. ’Nuff said, I love it!

Kudos Amazing speaker cabinet, microphone, and studio room simulations. Really good load box.
Concerns Required reboots on a couple of occasions.