Background: In our listening tests, we evaluated the Rx MK3-B with a wide variety of ancillary components as outlined below.
• Apple iPod Classic loaded exclusively with lossless CD-resolution files, with signals fed via high-quality line out dock (LOD) cable.
• Apple iPod Classic loaded exclusively with lossless CD-resolution files, with digital signal fed from the iPod to a Cypher Labs AlgoRhythm Solo portable DAC, with digital signals fed to the AlgoRhythm Solo and analog signals fed, respectively, through high-quality digital and analog cables from ALO Audio.
• Oppo BDP-95 universal/Blu-ray player, using the player’s balanced outputs with signals fed via high-quality balanced analog cables from Moon Audio.
Earphones and Custom-Fit In-Ear Monitors:
• Audeo by Phonak PFE 232
• Westone Elite Series ES5
• Ultimate Ears In-Ear Reference Monitors and Personal Reference Monitors.
• Audeze LCD3
• HiFiMAN HE-400 and HE-6
• Shure SRH1840
• Sennheiser HD800
The Sound: The ALO Rx MK3-B delivers overall sound quality that is fully comparable to that of an accomplished, full-size desktop amp. By this I mean that it shows all the hallmarks of larger amps: robust and well-controlled bass, authoritative and well-defined sound, and plenty of dynamic clout. What’s more, it also shows a thoroughly refined quality of smoothness so that even when reproducing small, low-level details it never exhibits the sharp, piercing, edginess that spoils the presentation of some transistor amps.
One area of particular strength is the entire bass region, where the ALO sounds taut and detailed, provides very fine low-frequency pitch definition, yet is also appropriately weighty, powerful, and punchy. Use the ALO to drive ‘phones with first-rate bass (e.g., the Audeze LCD-3 planar magnetic headphones) and you’ll find it easy to get lost in the sheer beauty and inner detail of low frequency instruments such as pipe organs, tympani, acoustic and electric basses, kick drums, etc.
Another welcome touch is the ALO’s Bass Boost control, which provides really subtle touches of low-end reinforcement without imposing a big, overblown, mid-bass sound the way some conventional “tone controls” do. The control can be a godsend for those who basically appreciate their current headphones, but yearn for a little more low-end weight. With the ALO, you can apply tasteful bass reinforcement at the twist of a control knob (or switch the bass boost circuit off altogether if that’s your preference).
Another key area of strength involves the Rx MK3-B’s midrange, which is highly transparent, well detailed, and highly dynamic, and yet consistently delivers that oh-so-elusive quality of refined smoothness I mentioned above. After listening to challenging orchestral passages first through the ALO and then through competing amps, I found the ALO was typically able to equal or, more likely, to surpass the performance of like-priced portable amplifiers in terms of the sheer amount of musical detail and nuance it could reproduce. When I tried this comparison between the Rx MK3-B and the also superb Ray Samuels SR-71B Blackbird, I found the outcomes were very close, that—to my ears—the ALO eked out a narrow but consistent performance edge over the SR-71B.
The bottom line is that the Rx MK3-B is one portable amp that sounds much bigger than it looks; it’s a pocket-sized powerhouse that can belt out music with serious gusto.
What’s the benefit of multiple gain settings? As mentioned above, the Rx MK3-B’s three gain-level settings allow users to match the amplifier's gain levels to fit the sensitivity of their chosen earphones or headphones. I found the best approach was to experiment by ear and then to choose the lowest gain settings that would still allow adequate volume levels with the earphones or headphones at hand—an approach that gives the best overall combination of low noise levels with satisfying volume levels. In practice, this meant I used the amp’s “Low” gain setting almost exclusively for high-sensitivity in-ear monitors, the “Medium” gain setting for the majority of listening scenarios, and the “High” gain setting in those instances where I needed/wanted to drive ultra-power-hungry headphones.
What about noise levels? I found the Rx MK3-B was absolutely quiet enough to use with high-sensitivity in-ear monitors, provided I first engaged the lowest of the amp’s three available gain settings. With the gain switch in the “Low” position, the ALO produced no discernible hiss or hum when used with either my Westone Elite Series ES5 custom-fit in-ear monitors or my Ultimate Ears Personal Reference Monitors. Indeed, the Rx MK3-B gave better results with the Westone monitors than other portable I’ve yet heard.