For me the USB input sounds best. But I will A/B again over the weekend... maybe.
Well, as promised I did some testing over the weekend. Didn't really have as much time as I hoped but will get back to it if there is interest.
Gear used:
Amp: MHA100 (duh)
Headphones: LCD-X
Cables: Norne Vanquish with Furutech FP-704 1/4", Kimber Kable USB B Bus Ag, Kabel Direkt Pro Series optical cable
Source: MacBook Pro (early 2011) with SSD and Audirvana Plus (no iTunes integration, stand-alone mode)
Settings: Normal 40 - 150 ohms @ 23% volume
Songs used:
Bat For Lashes - Glass [FLAC, 16/44.1]
Adele - Rolling In The Deep [MP3, 320kbps]
Lorde - Royals [FLAC, 24/48]
James Blake - Limit To Your Love [FLAC, 16/48]
James Blake - Retrograde [FLAC, 16/48]
I picked these songs because I know them very well and they each have elements that highlight certain strong or weak points of the audio chain.
I do not know nor care which part of the audio experience came from the cables, the DAC entry point, the MacBook or my ears. This is what I heard.
No tests have been done (yet) with different cables to confirm or disprove any findings.
When listening to
Glass by
Bat For Lashes the drums seemed to me to be more wide-spread and faster/punchier using the optical connection. There was quite a bit of air and a good feeling of space as well. Switching to USB the sound immediately struck me as warmer and darker. Bass was deeper and lasted longer, the fast drums began to partially melt into each other (as in, the first drum was still hanging in the air when the next one started). The soundstage seemed closer, both in width and depth. It was mellower and the mix itself felt more cohesive, whereas the optical connection offered more insight into the music and made it easier to distinguish each instrument or voice.
Next up was
Rolling In The Deep by
Adele and I was struck by how controlled the optical connection was. The big drum, the voice, Adele's voice... everything was so quick and clearly distinguishable. Again, switching to USB there was more warmth, the drums sounded fuller and had more low-down oomph, the voice sounded fuller, and the soundstage seemed smaller. I also think I heard a difference in quality between MP3 and FLAC. This may just have been my brain imagining things, though. Or the mix/production. Or maybe it is true and the soundstage is bigger on FLAC. Will most likely go back to this particular song in the near future.
Lorde's
Royals showed exactly the same picture with more control, wider soundstage, and feeling more detailed (though this might just be because it is brighter and emphasizes treble more) when using the optical output on the MacBook. USB gave that amazing depth to bass that is usually attributed to LCDs, made the whole sound warmer and the treble was just a hint smoother.
James Blake's two songs used here are both stunning examples for post-dubstep production and further confirmed the findings. Especially
Limit To Your Love with its pulsating (I won't say wobbling!) bass showcased the differences. Using the optical out it was fast, detailed and sufficiently deep. USB just gave the depth another dimension while giving up some of the speed. The treble over the optical connection was sharp and blisteringly fast, while over the USB connection it seemed smoother.
After reading all this I guess you already have a clear picture of the difference I heard or imagined when A/B-ing between optical and USB. I sincerely hope this comparison answers some questions, whereas for me the essential question is still not anywhere near answering: which one is better, optical or USB? I honestly cannot say as both have their advantages and it all depends on what you prefer and, maybe even more importantly, what kind of music you listen to. Treble-sensitive listeners may want to avoid the optical connection, based on my findings at least. The same could be said for bassheads. People looking for details and crisp treble will probably be more happy with the optical output. But, depending on cables used, headphones used, your ears and for all I can say the phase of the moon your mileage may vary significantly.
TL;DR:
Optical airier, crisper, faster, wider soundstage, more controlled.
USB warmer, lower extension, smoother treble, more cohesive feeling to the mix.