Pros: Superb transparent, natural and elegant sound, generous connectivity
Cons: Expensive, strains a bit with high impedance headphones, may be too accurate for some
After researching the crowded and occasionally confusing world of headphone amps, I chose the Grace Design m903 because of the generally positive critical reponse that it has engendered. My listening tastes run to acoustic music such as Classical and Jazz so an amp that "colors" the sound of headphones would not be desirable. The Grace Designs m903 is the latest addition to the well regarded line of deluxe headphone amps/DACS that began with the m901 around a decade ago. An important positive feature offered by the m903 are its generous connectivity options, enabling the user to easily mix-and-match components. It has USB connectivity for connections to a computer, as well as the full complement of currently available digital/analog connections to CD players, receivers and amplifiers. These options are apparently the broadest available, a fact that I discovered whilst researching headphone amps/DACS, in its price range. Set-up is simple and straight forward. A dial switches between connections, while a series of LEDs simultaneously indicate the sample rate of the digital source. The m903 supports sample rates up to 192 kHz on the MAC and 96kHz on the PC. Additional drivers which are offered for free download on the Grace Design website theoretically increase sample rates to 192 kHz on the PC but repeated attempts to install these drivers have not succeeded so I cannot vouch for their efficacy.
The m903's controls are precise and intuitive. Volume control occurs in steps of .05dB up to 99.5dB. You can have the m903 memorize your baseline volume setting so that it will begin there at start-up, eliminating the need for tedious dial twisting. The volume control has a built-in 3-step acceleration curve that enables faster access to higher dB settings. This is a very desirable feature, especially when using high impedance headphones such as the Beyerdynamic T1s or the Sennheiser 600/650/800s, which require volume settings in the 90dB range in order to be robustly driven. An X-Feed (crossfeed) feature simulates the signal crossfeed in the real acoustic space produced by speakers. The time delay and sound level differences that occur when the left ear hears music from the right speaker and vice-versa, as well as variations in frequency response produced as sound traverses the head, enable the brain to process the resulting aural image. These acoustic clues are missing when listening through headphones, causing instruments to appear to cluster in a particular segment of the sound stage. The crossfeed feature was created to simulate these missing psycho-acoustic cues, fooling the brain into imagining that it is experiencing a real acoustic space. This should lessen the risk of fatigue with prolonged listening, though the effect is difficult for some listeners to judge empirically. I noticed a perceptible difference in the sound stage and a slight improvement in instrumental focus. There is the possibility of a reduction in bass response with some musical sources when using this feature but the m903 effectively (and gently) compensates for this perceived loss.
Ultimately it is the quality of the music produced by the m903 that determines its value as a headphone amp/DAC and this is where it shines. It reproduces digital and analog sources with pristine transparency. It features a pleasing warmth that is acoustically neutral with no superfluous coloration. When listening to good sources through high end headphones there is a crisp and realistic tonal accuracy. Each instrument's timbre seems natural and well-rounded, heightening the sense of musical presence and the illusion of a real acoustic space. The words that best categorize the m903's sound are transparency, naturalness, elegance and accuracy. For some listeners who may be used to sound that is more "flavored" and "shaped", the m903 may be a bit too neutral and coldly accurate. If you prefer listening to music reproduced with a pronounced bias towards either treble or base, or a more forward mid-range, you may find the m903 a bit withdrawn or "veiled", a criticism that is occasionally aimed at Sennheiser headphones. Listening to high end Sennheiser's through the m903 will only heighten the sense of acoustic accuracy and neutrality of sound. However, if those are traits that appeal to you, the m903 is a superb choice.
The m903 comes with a 5 year limited warranty. Construction of the unit is solid and compact with all of the inputs and controls intelligently designed and implemented. A remote control is available but unfortunately it is not included. This is lamentable given the hefty price of the m903. However, a remote seems of limited functionality once you are up and running: volume is the most frequently encountered variable and that can be pre-set. If the acoustic attributes and features of the m903 appeal to you, and if the hefty asking price isn't too overwhelming, it is an excellent choice. The Grace Design m903 probably has the most features of any headphone amp/DAC in its price range.