• Sound Quality
• Ease of use
• Features
• Bang for buck
• Overall:

Short version - it makes a great converter sound even better.
Long version: While researching my most recent audio passion - HiRes audio (listening to recordings delivered in resolutions higher than 44.1k 16 bit 'CD quality'. I stumbled upon HiRes enthusiasts that were raving about this device so I felt I had to try it out.
I already have a high quality converter for listening to music playback, a Grace Designs M903 d/a converter and headphone amp.
I listen to the Grace via Sennheiser HD580 headphones and a pair of ADAM Artist 5 self powered monitors & ADAM Artist Sub unit.
Previously I just fed the Grace USB from my laptop.
I did my listening tests with various albums I have at 96 / 88.2 / 192k 24bit. I normally play these files back via Audirvana Plus software that connects to my Qobuz Sublime Plus account (allowing me to stream hi res audio via wifi) However switching quickly to change between USB to the Grace and Mutec re-clocked USB to the Grace was difficult with Audirvana so I used the Qobuz Mac app for playback. (Its also very good sounding although I feel Audirvana version 3.0.6 has the edge) This way I am able to switch fairly quickly between the converter receiving USB directly from my Macbook's USB output (via a USB C to USB 3 hub) and receiving re-clocked USB signal from the Mutec. (the Mutec receives the signal via USB 3 and sends it out into the Grace via AES 3 XLR)
So I just toggle the AES / USB input selector on the Grace M903 and also toggle the 'output device' selector within Qobuz to hear the difference.
Pop / Rock music listening
I have been enjoying The Essential Michael Jackson at 96k 24 bit. The tracked vocals are the first stand out in HiRes. The re-clocked USB signal via the Mutec sounds instantly 'wider'. Oddly enough it sounds a little 'softer' but thats a good quality because it sounds more 'lush'.
Listening to The Man in The Mirror I found I was suddenly following the bass line when I hadn't noticed it that much before. The kick drum sounds more 'deluxe' and classy via the Mutec and harder and less full range without.
All the different 'spaces' in the production become clearer, you can hear the different reverbs and depths of the (simulated) acoustic space in the production.
Things generally sound about 20-25% better with the Mutec. Spectacular vocals sounded MORE spectacular.
Classical / Baroque music listening
With the Mutec MC-3+ USB listening with headphones it felt that my hearing extended to the very outside edge of the headphones. When disconnected, the sound collapsed inwards to sound less wide and more next to my ear.
Cello and violin 'bow scraping' sound on the stringed Baroque chamber music sounds more present, lifelike and seems to have a more extended frequency responce, the same goes for harpsichord.
Over all I found that the Mutec MC-3+ USB supercharged my playback very nicely.
It also had one very unexpected and welcomed benefit. The Qobuz subscription I have streams HiRes over wifi and as the download progresses the playback can sometimes falter and drop out. This is very annoying but I tolerated it. Because the Mutec MC-3+ USB probably has a buffer in the chain for processing, the streaming dropouts never catch up to the output and get 'ironed out'! They have disappeared totally! For streaming 96k and 192k music that is fantastic, no more glitches!

Mutec manufacture high‑performance master clocks, and digital-format and sample‑rate converters. Everything is well designed, well engineered, and surprisingly cost effective. The MC3+ Smart Clock is a new‑generation product, which combines versatile master‑clock functionality (based on proprietary 1GHz internal clock technology) with a sophisticated and unusual audio re‑clocking function, which is designed to reduce jitter dramatically from a digital source. An external reference clock (including 10MHz atomic or GPS clocks) can also be redistributed through the MC3+, and the outputs are maintained in a fail‑safe way if the reference is lost.

The MC3+ is a 1U high, half‑rack unit with an internal universal power supply (90‑260 V AC, 10W). A mains on‑off switch is integrated with the IEC socket. External reference clocks are accepted via BNC connectors (wordclock, x256 Superclock, or 10MHz atomic/GPS), S/PDIF (coax and optical), or AES3/AES11. Five sets of clock outputs are arranged as three pairs of wordclock BNCs, S/PDIF (coax and optical), and AES3/AES11.

'MUTEC’s new visual identity marks an important step for the company communicating its current focus on the Hi-Fi consumer audio market. The MC-3+ Smart Clock USB with MUTEC’s sophisticated 1G-Clock and REVIVE Re-Clocking technologies has been received with much praise in the audiophile community.

Banks of LEDs indicate the current settings, starting with operating mode (internal clock, external clock, or re‑clock). The next LED groups indicate the selected external reference source, the current output sample rate, and the rate multipliers applied to each output group. The maximum output rate is restricted to 192kHz, plus x256 Superclock for the BNC outputs.

More LEDs on the right‑hand side indicate the current status (reference lock, fail‑safe output-rate hold, and digital audio present), and finally the master‑clock's working sample rate. Three oval LEDs on the left-hand side actually comprise a power‑on LED and two push‑buttons. The latter cycle through the set‑up options and select the required modes impressively: configuring the unit becomes entirely obvious within 10 seconds of pressing the buttons! Essentially, each press of the Menu button selects the next bank of LED indicators, whereupon the current selection flashes. Pressing the Select button then cycles through the various options, and the selected mode becomes active a few seconds after the last button press. Mathmagic pro edition indesign 9.41 for mac.

The internal and external clock modes are entirely conventional, albeit with excellent performancethanks to Mutec's bespoke 1G ultra‑low jitter technology. The Re‑clock mode allows an external digital audio signal to be phase‑aligned with an external reference clock (but not sample-rate converted), while simultaneously reducing source jitter dramatically. This can improve the audio quality substantially if employed between a jittery source and a D‑A converter susceptible to input jitter. Apparently many customers have bought the MC3+ for this purpose alone and report excellent results.

The MC3+ Smart Clock is a very smart clock indeed, with unusually flexible configurability, excellent technical performance, an attractive price, and a brilliantly simple user interface. Highly recommended. Hugh Robjohns

£559 including VAT.